Difference between revisions of "Area"

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[[File:Area.svg|right|thumb|alt=Three shapes on a square grid|The combined area of these three [[shapes]] is [[approximation|approximately]] 15.57 [[square]]s.]]
 
[[File:Area.svg|right|thumb|alt=Three shapes on a square grid|The combined area of these three [[shapes]] is [[approximation|approximately]] 15.57 [[square]]s.]]
'''Area''' is a [[quantity]] that expresses the extent of a [[two-dimensional]] [[surface]] or [[shape]], or [[planar lamina]], in the [[Plane (geometry)|plane]].  Area can be understood as the amount of material with a given thickness that would be necessary to fashion a model of the shape, or the amount of [[paint]] necessary to cover the surface with a single coat.<ref name=MathWorld>{{cite web|url=http://mathworld.wolfram.com/Area.html|title=Area|publisher=[[Wolfram MathWorld]]|author=[[Eric W. Weisstein]]|accessdate=3 July 2012}}</ref> It is the two-dimensional [[analogy|analog]] of the [[length]] of a [[plane curve|curve]] (a one-dimensional concept) or the [[volume]] of a [[solid geometry|solid]] (a three-dimensional concept).
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'''Area''' is the [[quantity]] that expresses the extent of a [[two-dimensional]] figure or [[shape]], or [[planar lamina]], in the [[Plane (geometry)|plane]]. [[Surface area]] is its analog on the two-dimensional [[surface]] of a [[solid geometry|three-dimensional object]]. Area can be understood as the amount of material with a given thickness that would be necessary to fashion a model of the shape, or the amount of [[paint]] necessary to cover the surface with a single coat.<ref name=MathWorld>{{cite web|url=http://mathworld.wolfram.com/Area.html|title=Area|publisher=[[Wolfram MathWorld]]|author=[[Eric W. Weisstein]]|accessdate=3 July 2012}}</ref> It is the two-dimensional [[analogy|analog]] of the [[length]] of a [[plane curve|curve]] (a one-dimensional concept) or the [[volume]] of a [[solid geometry|solid]] (a three-dimensional concept).
  
The area of a shape can be measured by comparing the shape to [[square]]s of a fixed size.<ref name=AF/> In the [[International System of Units]] (SI), the standard unit of area is the [[square metre]] (written as m<sup>2</sup>), which is the area of a square whose sides are one [[metre]] long.<ref name=B>[[Bureau International des Poids et Mesures]] [http://www.bipm.org/en/CGPM/db/11/12/ Resolution 12 of the 11th meeting of the CGPM (1960)], retrieved 15 July 2012</ref>  A shape with an area of three square metres would have the same area as three such squares.  In [[mathematics]], the [[unit square]] is defined to have area one, and the area of any other shape or surface is a [[Dimensionless quantity|dimensionless]] [[real number]].
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The area of a shape can be measured by comparing the shape to [[square]]s of a fixed size.<ref name=AF/> In the [[International System of Units]] (SI), the standard unit of area is the [[square metre]] (written as m<sup>2</sup>), which is the area of a square whose sides are one [[metre]] long.<ref name=B>[[Bureau International des Poids et Mesures]] [http://www.bipm.org/en/CGPM/db/11/12/ Resolution 12 of the 11th meeting of the CGPM (1960)], retrieved 15 July 2012</ref>  A shape with an area of three square metres would have the same area as three such squares.  In [[mathematics]], the [[unit square]] is defined to have area one, and the area of any other shape or surface is a [[Dimensionless quantity|dimensionless]] [[real number]].
  
 
There are several well-known [[formula]]s for the areas of simple shapes such as [[triangle]]s, [[rectangle]]s, and [[circle]]s.  Using these formulas, the area of any [[polygon]] can be found by [[Polygon triangulation|dividing the polygon into triangles]].<ref name=bkos>{{Cite book |author1=Mark de Berg |author2=Marc van Kreveld |author3=Mark Overmars |author3-link=Mark Overmars |author4=Otfried Schwarzkopf  |year=2000 |title=Computational Geometry |publisher=[[Springer-Verlag]] |edition=2nd revised |isbn=3-540-65620-0 |chapter=Chapter 3: Polygon Triangulation |pages=45–61 |postscript=<!-- Bot inserted parameter. Either remove it; or change its value to "." for the cite to end in a ".", as necessary. -->{{inconsistent citations}}}}</ref>  For shapes with curved boundary, [[calculus]] is usually required to compute the area.  Indeed, the problem of determining the area of plane figures was a major motivation for the [[History of calculus|historical development of calculus]].<ref>{{cite book|first=Carl B. |last=Boyer |authorlink=Carl Benjamin Boyer |title=A History of the Calculus and Its Conceptual Development |publisher=Dover |year=1959 |isbn=0-486-60509-4}}</ref>
 
There are several well-known [[formula]]s for the areas of simple shapes such as [[triangle]]s, [[rectangle]]s, and [[circle]]s.  Using these formulas, the area of any [[polygon]] can be found by [[Polygon triangulation|dividing the polygon into triangles]].<ref name=bkos>{{Cite book |author1=Mark de Berg |author2=Marc van Kreveld |author3=Mark Overmars |author3-link=Mark Overmars |author4=Otfried Schwarzkopf  |year=2000 |title=Computational Geometry |publisher=[[Springer-Verlag]] |edition=2nd revised |isbn=3-540-65620-0 |chapter=Chapter 3: Polygon Triangulation |pages=45–61 |postscript=<!-- Bot inserted parameter. Either remove it; or change its value to "." for the cite to end in a ".", as necessary. -->{{inconsistent citations}}}}</ref>  For shapes with curved boundary, [[calculus]] is usually required to compute the area.  Indeed, the problem of determining the area of plane figures was a major motivation for the [[History of calculus|historical development of calculus]].<ref>{{cite book|first=Carl B. |last=Boyer |authorlink=Carl Benjamin Boyer |title=A History of the Calculus and Its Conceptual Development |publisher=Dover |year=1959 |isbn=0-486-60509-4}}</ref>
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*32 Khata = 1 Acre
 
*32 Khata = 1 Acre
  
==Area formulae==
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==History==
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===Circle area===
  
===Polygon formulae===
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In the fifth century BCE, [[Hippocrates of Chios]] was the first to show that the area of a disk (the region enclosed by a circle) is proportional to the square of its diameter, as part of his quadrature of the [[lune of Hippocrates]],<ref name="heath">{{citation|first=Thomas L.|last=Heath|authorlink=Thomas Little Heath|title=A Manual of Greek Mathematics|publisher=Courier Dover Publications|year=2003|isbn=0-486-43231-9|pages=121–132|url=http://books.google.com/books?id=_HZNr_mGFzQC&pg=PA121}}.</ref> but did not identify the [[constant of proportionality]]. [[Eudoxus of Cnidus]], also in the fifth century BCE, also found that the area of a disk is proportional to its radius squared.<ref>{{cite book|last=Stewart|first=James|title=Single variable calculus early transcendentals.|year=2003|publisher=Brook/Cole|location=Toronto ON|isbn=0-534-39330-6|pages=3|url=http://www.stewartcalculus.com/media/8_home.php|edition=5th.|quote=However, by indirect reasoning, Eudoxus (fifth century B.C.) used exhaustion to prove the familiar formula for the area of a circle: <math>A= \pi r^2.</math>}} <!--This quote may be an overstatement. I have not been able to confirm that he discovered the actual formula, but perhaps only the proportionality between A and r-squared.--></ref>
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Subsequently, Book I of [[Euclid's Elements|Euclid's ''Elements'']] dealt with equality of areas between two-dimensional figures. The mathematician [[Archimedes]] used the tools of [[Euclidean geometry]] to show that the area inside a circle is equal to that of a [[right triangle]] whose base has the length of the circle's circumference and whose height equals the circle's radius, in his book ''[[Measurement of a Circle]]''. (The circumference is 2{{pi}}''r'', and the area of a triangle is half the base times the height, yielding the area {{pi}}''r''<sup>2</sup> for the disk.) Archimedes approximated the value of π (and hence the area of a unit-radius circle) with [[Area of a disk#Archimedes' doubling method|his doubling method]], in which he inscribed a regular triangle in a circle and noted its area, then doubled the number of sides to give a regular [[hexagon]], then repeatedly doubled the number of sides as the polygon's area got closer and closer to that of the circle (and did the same with [[circumscribed polygon]]s).
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Swiss scientist [[Johann Heinrich Lambert]] in 1761 proved that [[pi|π]], the ratio of a circle's area to its squared radius, is [[irrational number|irrational]], meaning it is not equal to the quotient of any two whole numbers.<ref name=Arndt>{{cite book|last=Arndt|first=Jörg|last2=Haenel|first2=Christoph|title=Pi Unleashed|publisher=Springer-Verlag|year=2006|isbn=978-3-540-66572-4 <!--isbn only volume 1-->|url=http://books.google.com/?id=QwwcmweJCDQC&printsec=frontcover#v=onepage&q&f=false|ref=harv|accessdate=2013-06-05}} English translation by Catriona and David Lischka.</ref> French mathematician [[Adrien-Marie Legendre]] proved in 1794 that π<sup>2</sup> is also irrational. In 1882, German mathematician [[Ferdinand von Lindemann]] proved that π is [[transcendental number|transcendental]] (not the solution of any [[polynomial equation]] with rational coefficients), confirming a conjecture made by both [[Legendre]] and Euler.<ref name=Arndt/>{{rp|p. 196}}
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===Triangle area===
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[[Hero of Alexandria|Heron (or Hero) of Alexandria]] found what is known as [[Heron's formula]] for the area of a triangle in terms of its sides, and a proof can be found in his book, ''Metrica'', written around 60 CE. It has been suggested that [[Archimedes]] knew the formula over two centuries earlier,<ref>{{cite book
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| author=Heath, Thomas L.
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| title=A History of Greek Mathematics (Vol II)
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| publisher=Oxford University Press
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| year=1921
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| pages=321–323}}</ref> and since ''Metrica'' is a collection of the mathematical knowledge available in the ancient world, it is possible that the formula predates the reference given in that work.<ref>{{MathWorld |urlname=HeronsFormula |title=Heron's Formula}}</ref>
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In 499 [[Aryabhata]], a great [[mathematician]]-[[astronomer]] from the classical age of [[Indian mathematics]] and [[Indian astronomy]], expressed the area of a triangle as one-half the base times the height in the ''[[Aryabhatiya]]'' (section 2.6).
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A formula equivalent to Heron's was discovered by the Chinese independently of the Greeks. It was published in 1247 in ''Shushu Jiuzhang'' (“[[Mathematical Treatise in Nine Sections]]”), written by [[Qin Jiushao]].
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===Quadrilateral area===
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In the 600s CE, [[Brahmagupta]] developed a formula, now known as [[Brahmagupta's formula]], for the area of a [[cyclic quadrilateral]] (a [[quadrilateral]] [[inscribed figure|inscribed]] in a circle) in terms of its sides. In 1842 the German mathematicians [[Carl Anton Bretschneider]] and [[Karl Georg Christian von Staudt]] independently found a formula, known as [[Bretschneider's formula]], for the area of any quadrilateral.
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===General polygon area===
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The development of [[Cartesian coordinates]] by [[René Descartes]] in the 1600s allowed the development of the [[Shoelace formula|surveyor's formula]] for the area of any polygon with known [[vertex (geometry)|vertex]] locations by [[Gauss]] in the 1800s.
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===Areas determined using calculus===
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The development of [[integral calculus]] in the late 1600s provided tools that could subsequently be used for computing more complicated areas, such as the area of an [[ellipse#Area|ellipse]] and the [[surface area]]s of various curved three-dimensional objects.
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==Area formulas==
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===Polygon formulas===
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{{Main|Polygon#Area and centroid}}
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For a non-self-intersecting ([[simple polygon|simple]]) polygon, the [[Cartesian coordinates]] <math>(x_i,  y_i)</math> (''i''=0, 1, ..., ''n''-1) of whose ''n'' [[vertex (geometry)|vertices]] are known, the area is given by the [[shoelace formula|surveyor's formula]]:<ref>{{cite web
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|url        = http://www.seas.upenn.edu/~sys502/extra_materials/Polygon%20Area%20and%20Centroid.pdf
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|title      = Calculating The Area And Centroid Of A Polygon
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|last      = Bourke
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|first      = Paul
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|date      = July 1988
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|work      =
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|publisher  =
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|accessdate = 6 Feb 2013
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}}
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</ref>
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:<math>A = \frac{1}{2} | \sum_{i = 0}^{n - 1}( x_i y_{i + 1} - x_{i + 1} y_i) |</math>
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where when ''i''=''n''-1, then ''i''+1 is expressed as [[modular arithmetic|modulus]] ''n'' and so refers to 0.
  
 
====Rectangles====
 
====Rectangles====
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[[Image:ParallelogramArea.svg|thumb|left|180px|alt=A diagram showing how a parallelogram can be re-arranged into the shape of a rectangle|Equal area figures.]]
 
[[Image:ParallelogramArea.svg|thumb|left|180px|alt=A diagram showing how a parallelogram can be re-arranged into the shape of a rectangle|Equal area figures.]]
  
====Dissection formulae====
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====Dissection, parallelograms, and triangles====
Most other simple formulae for area follow from the method of [[dissection (geometry)|dissection]].
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{{Main|Triangle#Computing the area of a triangle|Parallelogram#Area formula}}
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Most other simple formulas for area follow from the method of [[dissection (geometry)|dissection]].
 
This involves cutting a shape into pieces, whose areas must [[addition|sum]] to the area of the original shape.
 
This involves cutting a shape into pieces, whose areas must [[addition|sum]] to the area of the original shape.
  
 
For an example, any [[parallelogram]] can be subdivided into a [[trapezoid]] and a [[right triangle]], as shown in figure to the left.  If the triangle is moved to the other side of the trapezoid, then the resulting figure is a rectangle.  It follows that the area of the parallelogram is the same as the area of the rectangle:<ref name=AF/>
 
For an example, any [[parallelogram]] can be subdivided into a [[trapezoid]] and a [[right triangle]], as shown in figure to the left.  If the triangle is moved to the other side of the trapezoid, then the resulting figure is a rectangle.  It follows that the area of the parallelogram is the same as the area of the rectangle:<ref name=AF/>
 
:{{bigmath|''A'' {{=}} ''bh''}} <big>&nbsp;(parallelogram).</big>
 
:{{bigmath|''A'' {{=}} ''bh''}} <big>&nbsp;(parallelogram).</big>
[[Image:TriangleArea.svg|thumb|right|180px|alt=A parallelogram split into two equal triangles|Two equal triangles.]]However, the same parallelogram can also be cut along a [[diagonal]] into two [[congruence (geometry)|congruent]] triangles, as shown in the figure to the right.  It follows that the area of each triangle is half the area of the parallelogram:<ref name=AF/>  
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[[Image:TriangleArea.svg|thumb|right|180px|alt=A parallelogram split into two equal triangles|Two equal triangles.]]However, the same parallelogram can also be cut along a [[diagonal]] into two [[congruence (geometry)|congruent]] triangles, as shown in the figure to the right.  It follows that the area of each [[triangle]] is half the area of the parallelogram:<ref name=AF/>  
 
:<math>A = \frac{1}{2}bh</math> <big>&nbsp;(triangle).</big>
 
:<math>A = \frac{1}{2}bh</math> <big>&nbsp;(triangle).</big>
Similar arguments can be used to find area formulae for the [[trapezoid]] and the [[rhombus]], as well as more complicated [[polygon]]s.{{citation needed|date=July 2012}}
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Similar arguments can be used to find area formulas for the [[trapezoid]]<ref>{{citation|title=Problem Solving Through Recreational Mathematics|first1=Bonnie|last1=Averbach|first2=Orin|last2=Chein|publisher=Dover|year=2012|isbn=9780486131740|page=306|url=http://books.google.com/books?id=Dz_CAgAAQBAJ&pg=PA306}}.</ref> as well as more complicated [[polygon]]s.<ref>{{citation|title=Calculus for Scientists and Engineers: An Analytical Approach|first=K. D.|last=Joshi|publisher=CRC Press|year=2002|isbn=9780849313196|page=43|url=http://books.google.com/books?id=5SDcLHkelq4C&pg=PA43}}.</ref>
  
 
===Area of curved shapes===
 
===Area of curved shapes===
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{{main|Surface area}}
 
{{main|Surface area}}
 
[[Image:Archimedes sphere and cylinder.svg|right|thumb|180px|alt=A blue sphere inside a cylinder of the same height and radius|[[Archimedes]] showed that the surface area and volume of a [[sphere]] is exactly 2/3 of the area and volume of the surrounding [[cylinder (geometry)|cylindrical]] surface.]]
 
[[Image:Archimedes sphere and cylinder.svg|right|thumb|180px|alt=A blue sphere inside a cylinder of the same height and radius|[[Archimedes]] showed that the surface area and volume of a [[sphere]] is exactly 2/3 of the area and volume of the surrounding [[cylinder (geometry)|cylindrical]] surface.]]
Most basic formulae for [[surface area]] can be obtained by cutting surfaces and flattening them out.  For example, if the side surface of a [[cylinder (geometry)|cylinder]] (or any [[prism (geometry)|prism]]) is cut lengthwise, the surface can be flattened out into a rectangle.  Similarly, if a cut is made along the side of a [[cone (geometry)|cone]], the side surface can be flattened out into a sector of a circle, and the resulting area computed.
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Most basic formulas for [[surface area]] can be obtained by cutting surfaces and flattening them out.  For example, if the side surface of a [[cylinder (geometry)|cylinder]] (or any [[prism (geometry)|prism]]) is cut lengthwise, the surface can be flattened out into a rectangle.  Similarly, if a cut is made along the side of a [[cone (geometry)|cone]], the side surface can be flattened out into a sector of a circle, and the resulting area computed.
  
 
The formula for the surface area of a [[sphere]] is more difficult to derive: because a sphere has nonzero [[Gaussian curvature]], it cannot be flattened out.  The formula for the surface area of a sphere was first obtained by [[Archimedes]] in his work ''[[On the Sphere and Cylinder]]''.  The formula is:<ref name=MathWorldSurfaceArea/>
 
The formula for the surface area of a [[sphere]] is more difficult to derive: because a sphere has nonzero [[Gaussian curvature]], it cannot be flattened out.  The formula for the surface area of a sphere was first obtained by [[Archimedes]] in his work ''[[On the Sphere and Cylinder]]''.  The formula is:<ref name=MathWorldSurfaceArea/>
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where {{math|''r''}} is the radius of the sphere.  As with the formula for the area of a circle, any derivation of this formula inherently uses methods similar to [[calculus]].
 
where {{math|''r''}} is the radius of the sphere.  As with the formula for the area of a circle, any derivation of this formula inherently uses methods similar to [[calculus]].
  
===General formulae===
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===General formulas===
  
 
====Areas of 2-dimensional figures====
 
====Areas of 2-dimensional figures====
*A [[triangle]]: <math>\tfrac12Bh</math> (where ''B'' is any side, and ''h'' is the distance from the line on which ''B'' lies to the other vertex of the triangle). This formula can be used if the height ''h'' is known. If the lengths of the three sides are known then ''[[Heron's formula]]'' can be used: <math>\sqrt{s(s-a)(s-b)(s-c)}</math> where ''a'', ''b'', ''c'' are the sides of the triangle, and <math>s = \tfrac12(a + b + c)</math> is half of its perimeter.<ref name=AF/> If an angle and its two included sides are given, the area is <math>\tfrac12 a b \sin(C)</math> where {{math|''C''}} is the given angle and {{math|''a''}} and {{math|''b''}} are its included sides.<ref name=AF/> If the triangle is graphed on a coordinate plane, a matrix can be used and is simplified to the absolute value of <math>\tfrac12(x_1 y_2 + x_2 y_3 + x_3 y_1 - x_2 y_1 - x_3 y_2 - x_1 y_3)</math>. This formula is also known as the [[shoelace formula]] and is an easy way to solve for the area of a coordinate triangle by substituting the 3 points ''(x<sub>1</sub>,y<sub>1</sub>)'', ''(x<sub>2</sub>,y<sub>2</sub>)'', and ''(x<sub>3</sub>,y<sub>3</sub>)''. The shoelace formula can also be used to find the areas of other polygons when their vertices are known. Another approach for a coordinate triangle is to use [[Infinitesimal calculus]] to find the area.
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*A [[triangle]]: <math>\tfrac12Bh</math> (where ''B'' is any side, and ''h'' is the distance from the line on which ''B'' lies to the other vertex of the triangle). This formula can be used if the height ''h'' is known. If the lengths of the three sides are known then ''[[Heron's formula]]'' can be used: <math>\sqrt{s(s-a)(s-b)(s-c)}</math> where ''a'', ''b'', ''c'' are the sides of the triangle, and <math>s = \tfrac12(a + b + c)</math> is half of its perimeter.<ref name=AF/> If an angle and its two included sides are given, the area is <math>\tfrac12 a b \sin(C)</math> where {{math|''C''}} is the given angle and {{math|''a''}} and {{math|''b''}} are its included sides.<ref name=AF/> If the triangle is graphed on a coordinate plane, a matrix can be used and is simplified to the absolute value of <math>\tfrac12(x_1 y_2 + x_2 y_3 + x_3 y_1 - x_2 y_1 - x_3 y_2 - x_1 y_3)</math>. This formula is also known as the [[shoelace formula]] and is an easy way to solve for the area of a coordinate triangle by substituting the 3 points ''(x<sub>1</sub>,y<sub>1</sub>)'', ''(x<sub>2</sub>,y<sub>2</sub>)'', and ''(x<sub>3</sub>,y<sub>3</sub>)''. The shoelace formula can also be used to find the areas of other polygons when their vertices are known. Another approach for a coordinate triangle is to use [[calculus]] to find the area.
 
*A [[simple polygon]] constructed on a grid of equal-distanced points (i.e., points with [[integer]] coordinates) such that all the polygon's vertices are grid points: <math>i + \frac{b}{2} - 1</math>, where ''i'' is the number of grid points inside the polygon and ''b'' is the number of boundary points.<ref name=Pick>{{cite journal |last=Trainin |first=J. |title=An elementary proof of Pick's theorem |journal=[[Mathematical Gazette]] |volume=91 |issue=522 |date=November 2007 |pages=536–540}}</ref> This result is known as [[Pick's theorem]].<ref name=Pick/>
 
*A [[simple polygon]] constructed on a grid of equal-distanced points (i.e., points with [[integer]] coordinates) such that all the polygon's vertices are grid points: <math>i + \frac{b}{2} - 1</math>, where ''i'' is the number of grid points inside the polygon and ''b'' is the number of boundary points.<ref name=Pick>{{cite journal |last=Trainin |first=J. |title=An elementary proof of Pick's theorem |journal=[[Mathematical Gazette]] |volume=91 |issue=522 |date=November 2007 |pages=536–540}}</ref> This result is known as [[Pick's theorem]].<ref name=Pick/>
  
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::<math> \oint_{t_0}^{t_1} x \dot y \, dt  = - \oint_{t_0}^{t_1} y \dot x \, dt  =  {1 \over 2} \oint_{t_0}^{t_1} (x \dot y - y \dot x) \, dt </math>
 
::<math> \oint_{t_0}^{t_1} x \dot y \, dt  = - \oint_{t_0}^{t_1} y \dot x \, dt  =  {1 \over 2} \oint_{t_0}^{t_1} (x \dot y - y \dot x) \, dt </math>
 
(see [[Green's theorem]]) or the ''z''-component of
 
(see [[Green's theorem]]) or the ''z''-component of
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:<math>{1 \over 2} \oint_{t_0}^{t_1} \vec u \times \dot{\vec u} \, dt.</math>
  
:<math>{1 \over 2} \oint_{t_0}^{t_1} \vec u \times \dot{\vec u} \, dt.</math>
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====Bounded area between two quadratic functions====
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To find the bounded area between two [[quadratic function]]s, we subtract one from the other to write the difference as
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:<math>f(x)-g(x)=ax^2+bx+c</math>
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where ''f''(''x'') is the quadratic upper bound and ''g''(''x'') is the quadratic lower bound. Define the [[discriminant]] of ''f''(''x'')-''g''(''x'') as
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:<math>\Delta=b^2-4ac.</math>
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By simplifying the integral formula between the graphs of two functions (as given in the section above) and using [[Vieta's formulas|Vieta's formula]], we can obtain<ref>[http://books.google.com/books?id=NFkVfrZBqpUC&pg=PA51&redir_esc=y#v=onepage&q&f=false Matematika - Marthen Kanginan], Google Books.</ref><ref>[http://books.google.com/books?id=uwqvITs8OaUC&pg=PA157&redir_esc=y#v=onepage&q&f=false Get Success UN +SPMB Matematika - Juliah], Google Books.</ref>
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:<math>A=\frac{\Delta\sqrt{\Delta}}{6a^2},\qquad a\neq0.</math>
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The above remains valid if one of the bounding functions is linear instead of quadratic.
  
 
====Surface area of 3-dimensional figures====
 
====Surface area of 3-dimensional figures====
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*[[rectangular prism]]: <math>2 (\ell w + \ell  h + w h)</math>, where <math>\ell</math> is the length, ''w'' is the width, and ''h'' is the height.
 
*[[rectangular prism]]: <math>2 (\ell w + \ell  h + w h)</math>, where <math>\ell</math> is the length, ''w'' is the width, and ''h'' is the height.
  
====General formula====
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====General formula for surface area====
 
The general formula for the surface area of the graph of a continuously differentiable function <math>z=f(x,y),</math> where <math>(x,y)\in D\subset\mathbb{R}^2</math> and <math>D</math> is a region in the xy-plane with the smooth boundary:
 
The general formula for the surface area of the graph of a continuously differentiable function <math>z=f(x,y),</math> where <math>(x,y)\in D\subset\mathbb{R}^2</math> and <math>D</math> is a region in the xy-plane with the smooth boundary:
 
: <math> A=\iint_D\sqrt{\left(\frac{\partial f}{\partial x}\right)^2+\left(\frac{\partial f}{\partial y}\right)^2+1}\,dx\,dy. </math>
 
: <math> A=\iint_D\sqrt{\left(\frac{\partial f}{\partial x}\right)^2+\left(\frac{\partial f}{\partial y}\right)^2+1}\,dx\,dy. </math>
Even more general formula for the area of the graph of a [[parametric surface]] in the vector form <math>\mathbf{r}=\mathbf{r}(u,v),</math> where <math>\mathbf{r}</math> is a continuously differentiable vector function of <math>(u,v)\in D\subset\mathbb{R}^2</math>:<ref name="doCarmo"/>
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An even more general formula for the area of the graph of a [[parametric surface]] in the vector form <math>\mathbf{r}=\mathbf{r}(u,v),</math> where <math>\mathbf{r}</math> is a continuously differentiable vector function of <math>(u,v)\in D\subset\mathbb{R}^2</math> is:<ref name="doCarmo"/>
 
: <math> A=\iint_D \left|\frac{\partial\mathbf{r}}{\partial u}\times\frac{\partial\mathbf{r}}{\partial v}\right|\,du\,dv. </math>
 
: <math> A=\iint_D \left|\frac{\partial\mathbf{r}}{\partial u}\times\frac{\partial\mathbf{r}}{\partial v}\right|\,du\,dv. </math>
  
 
===List of formulas===
 
===List of formulas===
<!-- NOTICE TO CONTRIBUTORS
 
This section is regularly edited to change "formulae" to "formula". "Formula" is the singular, whilst "formulae" a plural form. Since there are multiple formulae listed, "formula" is incorrect in this case. Thank you for your understanding.
 
Please raise any further issues you may have on the talk page.-->
 
 
There are [[formula]]e for many different regular and irregular polygons, and those additional to the ones above are listed here.
 
  
 
{| class="wikitable"
 
{| class="wikitable"
|+ Additional common formulae for area:
+
|+ Additional common formulas for area:
 
! Shape
 
! Shape
 
! Formula
 
! Formula
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|-
 
|-
 
|Regular [[triangle]] ([[equilateral triangle]])
 
|Regular [[triangle]] ([[equilateral triangle]])
||<math>\frac\sqrt{3}{4}s^2\,\!</math>
+
||<math>\frac{\sqrt{3}}{4}s^2\,\!</math>
 
||<math>s</math> is the length of one side of the triangle.
 
||<math>s</math> is the length of one side of the triangle.
 
|-
 
|-
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|-
 
|-
 
|[[Isosceles triangle]]
 
|[[Isosceles triangle]]
|<math>\frac{1}{2}b\sqrt{a^2-\frac{b^2}{4}}</math>
+
|<math>\frac{1}{2}b\sqrt{a^2-\frac{b^2}{4}}=\frac{b}{4}\sqrt{4a^2-b^2}</math>
|<math>a</math> is the length of an equal side and <math>b</math> is the length of a different side.
+
|<math>a</math> is the length of one of the two equal sides and <math>b</math> is the length of a different side.
 
|-
 
|-
|[[Rhombus]]
+
|[[Rhombus]]/[[Kite (geometry)|Kite]]
 
|<math>\tfrac12ab</math>
 
|<math>\tfrac12ab</math>
|<math>a</math> and <math>b</math> are the lengths of the two [[diagonals]] of the rhombus.
+
|<math>a</math> and <math>b</math> are the lengths of the two [[diagonals]] of the rhombus or kite.
 
|-
 
|-
 
|[[Parallelogram]]
 
|[[Parallelogram]]
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|-
 
|-
 
|[[Circle|Circular]] to square area conversion
 
|[[Circle|Circular]] to square area conversion
|<math>\frac{1}{4} C\pi\,\!</math>
+
|<math>\frac{\pi}{4} C\,\!</math>
 
|<math>C</math> is the area of the [[circle]] in circular units.
 
|<math>C</math> is the area of the [[circle]] in circular units.
|-
 
|[[Reuleaux Triangle]]
 
|<math>\frac{\pi x^2}{6}-\frac{3 \sqrt{x^2-(\frac{x}{2})^2}}{2}+\frac{\sqrt{x^2-(\frac{x}{2})^2}}{2}</math>
 
|<math>x</math> is the side of the triangle inside the reuleaux triangle.
 
 
<!--
 
<!--
 
|-
 
|-
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|}
 
|}
  
The above calculations show how to find the area of many common [[shapes]].
+
The above calculations show how to find the areas of many common [[shapes]].
  
 
The areas of irregular polygons can be calculated using the "[[Surveyor's formula]]".<ref name=Surveyor>{{cite journal|last1=Braden|first1=Bart|date=September 1986|title= The Surveyor's Area Formula|journal=The College Mathematics Journal|volume=17|issue=4|pages=326–337|publisher=|doi=10.2307/2686282|url=http://www.maa.org/pubs/Calc_articles/ma063.pdf|accessdate=15 July 2012}}</ref>
 
The areas of irregular polygons can be calculated using the "[[Surveyor's formula]]".<ref name=Surveyor>{{cite journal|last1=Braden|first1=Bart|date=September 1986|title= The Surveyor's Area Formula|journal=The College Mathematics Journal|volume=17|issue=4|pages=326–337|publisher=|doi=10.2307/2686282|url=http://www.maa.org/pubs/Calc_articles/ma063.pdf|accessdate=15 July 2012}}</ref>
 +
 +
===Relation of area to perimeter===
 +
 +
The [[isoperimetric inequality]] states that, for a closed curve of length ''L'' (so the region it encloses has [[perimeter]] ''L'') and for area ''A'' of the region that it encloses,
 +
 +
:<math>4\pi A \le L^2,</math>
 +
 +
and  equality holds if and only if the curve is a [[circle]]. Thus a circle has the largest area of any closed figure with a given perimeter.
 +
 +
At the other extreme, a figure with given perimeter ''L'' could have an arbitrarily small area, as illustrated by a [[rhombus]] that is "tipped over" arbitrarily far so that two of its [[angle]]s are arbitrarily close to 0° and the other two are arbitrarily close to 180°.
 +
 +
For a circle, the ratio of the area to the [[circumference]] (the term for the perimeter of a circle) equals half the [[radius]] ''r''. This can be seen from the area formula ''πr''<sup>2</sup> and the circumference formula 2''πr''.
 +
 +
The area of a [[regular polygon]] is half its perimeter times the [[apothem]] (where the apothem is the distance from the center to the nearest point on any side).
 +
 +
===Fractals===
 +
 +
Doubling the edge lengths of a polygon multiplies its area by four, which is two (the ratio of the new to the old side length) raised to the power of two (the dimension of the space the polygon resides in). But if the one-dimensional lengths of a  [[fractal]] drawn in two dimensions are all doubled, the spatial content of the fractal scales by a power of two that is not necessarily an integer. This power is called the [[fractal dimension]] of the fractal.
 +
<ref name="Mandelbrot1983">
 +
{{cite book
 +
|last=Mandelbrot |first=Benoît B.
 +
|title=The fractal geometry of nature
 +
|url=http://books.google.com/books?id=0R2LkE3N7-oC
 +
|accessdate=1 February 2012
 +
|year=1983
 +
|publisher=Macmillan
 +
|isbn=978-0-7167-1186-5}}</ref>
 +
 +
==Area bisectors==
 +
{{Main|Bisection#Area bisectors and perimeter bisectors}}
 +
 +
There are an infinitude of lines that bisect the area of a triangle. Three of them are the [[Median (triangle)|medians]] of the triangle (which connect the sides' midpoints with the opposite vertices), and these are [[Concurrent lines|concurrent]] at the triangle's [[centroid]]; indeed, they are the only area bisectors that go through the centroid. Any line through a triangle that splits both the triangle's area and its perimeter in half goes through the triangle's incenter (the center of its [[incircle]]). There are either one, two, or three of these for any given triangle.
 +
 +
Any line through the midpoint of a parallelogram bisects the area.
 +
 +
All area bisectors of a circle or other ellipse go through the center, and any [[Chord (geometry)|chords]] through the center bisect the area. In the case of a circle they are the diameters of the circle.
  
 
==Optimization==
 
==Optimization==
 
Given a wire contour, the surface of least area spanning ("filling") it is a [[minimal surface]].  Familiar examples include [[soap bubble]]s.
 
Given a wire contour, the surface of least area spanning ("filling") it is a [[minimal surface]].  Familiar examples include [[soap bubble]]s.
  
The question of the [[filling area conjecture|filling area]] of the [[Riemannian circle]] remains open.{{citation needed|date=October 2012}}
+
The question of the [[filling area conjecture|filling area]] of the [[Riemannian circle]] remains open.<ref>{{citation
 +
| last = Gromov | first = Mikhael
 +
| issue = 1
 +
| journal = Journal of Differential Geometry
 +
| mr = 697984
 +
| pages = 1–147
 +
| title = Filling Riemannian manifolds
 +
| url = http://projecteuclid.org/euclid.jdg/1214509283
 +
| volume = 18
 +
| year = 1983}}.</ref>
 +
 
 +
The circle has the largest area of any two-dimensional object having the same perimeter.
 +
 
 +
A [[cyclic polygon]] (one inscribed in a circle) has the largest area of any polygon with a given number of sides of the same lengths.
 +
 
 +
A version of the [[isoperimetric inequality]] for triangles states that the triangle of greatest area among all those with a given perimeter is [[equilateral]].<ref name=Chakerian/>
 +
 
 +
The triangle of largest area of all those inscribed in a given circle is equilateral; and the triangle of smallest area of all those circumscribed around a given circle is equilateral.<ref>Dorrie, Heinrich, ''100 Great Problems of Elementary Mathematics'', Dover Publ., 1965: 379-380.</ref>
 +
 
 +
The ratio of the area of the incircle to the area of an equilateral triangle, <math>\frac{\pi}{3\sqrt{3}}</math>, is larger than that of any non-equilateral triangle.<ref>Minda, D., and Phelps, S., "Triangles, ellipses, and cubic polynomials", ''[[American Mathematical Monthly]]'' 115, October 2008, 679-689: Theorem 4.1.</ref>
 +
 
 +
The ratio of the area to the square of the perimeter of an equilateral triangle, <math>\frac{1}{12\sqrt{3}},</math> is larger than that for any other triangle.<ref name=Chakerian>Chakerian, G. D. "A Distorted View of Geometry." Ch. 7 in ''Mathematical Plums'' (R. Honsberger, editor). Washington, DC: Mathematical Association of America, 1979: 147.</ref>
  
 
==See also==
 
==See also==
 +
*[[Brahmagupta quadrilateral]], a cyclic quadrilateral with integer sides, integer diagonals, and integer area.
 
*[[2 × 2 real matrices#Equi-areal mapping|Equi-areal mapping]]
 
*[[2 × 2 real matrices#Equi-areal mapping|Equi-areal mapping]]
*[[Integral]]
+
*[[Heron triangle]], a triangle with integer sides and integer area.
 +
*[[List of triangle inequalities#Area]]
 +
*[[One-seventh area triangle]], an inner triangle with one-seventh the area of the reference triangle.
 +
:*[[Routh's theorem]], a generalization of the one-seventh area triangle.
 
*[[Orders of magnitude (area)]]&mdash;A list of areas by size.
 
*[[Orders of magnitude (area)]]&mdash;A list of areas by size.
*[[Perimeter]]
+
*[[Pentagon#Derivation of the area formula]]
 
*[[Planimeter]], an instrument for measuring small areas, e.g. on maps.
 
*[[Planimeter]], an instrument for measuring small areas, e.g. on maps.
 +
*[[Quadrilateral#Area of a convex quadrilateral]]
 +
*[[Robbins pentagon]], a cyclic pentagon whose side lengths and area are all rational numbers.
 
*[[Volume]]
 
*[[Volume]]
  
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==External links==
 
==External links==
 +
{{Sisterlinks|d=Q11500|commons=Category:Area|wikt=area|b=no|v=no|voy=no|species=no|mw=no|m=no|n=no|s=no|q=no}}
 
{{Commons category|Area}}
 
{{Commons category|Area}}
 
{{Wiktionary}}
 
{{Wiktionary}}
 
* [http://www.area-of-a-circle.com Area Calculator]
 
* [http://www.area-of-a-circle.com Area Calculator]
 +
* [http://www.fxsolver.com/browse/?q=area+of&search=&formulas=on Area Equations Graphing Calculator]
 
* [http://www.sengpielaudio.com/calculator-cross-section.htm Conversion cable diameter to circle cross-sectional area and vice versa]
 
* [http://www.sengpielaudio.com/calculator-cross-section.htm Conversion cable diameter to circle cross-sectional area and vice versa]
 
* [http://milloz.com/site/index.php?q=Free-Tools/Area-Measurement-Tool Geographical Area Calculator using Satellite Maps View]
 
* [http://milloz.com/site/index.php?q=Free-Tools/Area-Measurement-Tool Geographical Area Calculator using Satellite Maps View]
  
[[Category:Area|*]]
+
[[Category:Area| ]]

Latest revision as of 03:25, 25 December 2014

{{#invoke:Hatnote|hatnote}}

Three shapes on a square grid
The combined area of these three shapes is approximately 15.57 squares.

Area is the quantity that expresses the extent of a two-dimensional figure or shape, or planar lamina, in the plane. Surface area is its analog on the two-dimensional surface of a three-dimensional object. Area can be understood as the amount of material with a given thickness that would be necessary to fashion a model of the shape, or the amount of paint necessary to cover the surface with a single coat.[1] It is the two-dimensional analog of the length of a curve (a one-dimensional concept) or the volume of a solid (a three-dimensional concept).

The area of a shape can be measured by comparing the shape to squares of a fixed size.[2] In the International System of Units (SI), the standard unit of area is the square metre (written as m2), which is the area of a square whose sides are one metre long.[3] A shape with an area of three square metres would have the same area as three such squares. In mathematics, the unit square is defined to have area one, and the area of any other shape or surface is a dimensionless real number.

There are several well-known formulas for the areas of simple shapes such as triangles, rectangles, and circles. Using these formulas, the area of any polygon can be found by dividing the polygon into triangles.[4] For shapes with curved boundary, calculus is usually required to compute the area. Indeed, the problem of determining the area of plane figures was a major motivation for the historical development of calculus.[5]

For a solid shape such as a sphere, cone, or cylinder, the area of its boundary surface is called the surface area.[1][6] Formulas for the surface areas of simple shapes were computed by the ancient Greeks, but computing the surface area of a more complicated shape usually requires multivariable calculus.

Area plays an important role in modern mathematics. In addition to its obvious importance in geometry and calculus, area is related to the definition of determinants in linear algebra, and is a basic property of surfaces in differential geometry.[7] In analysis, the area of a subset of the plane is defined using Lebesgue measure,[8] though not every subset is measurable.[9] In general, area in higher mathematics is seen as a special case of volume for two-dimensional regions.[1]

Area can be defined through the use of axioms, defining it as a function of a collection of certain plane figures to the set of real numbers. It can be proved that such a function exists.

Formal definition

{{#invoke:see also|seealso}} An approach to defining what is meant by "area" is through axioms. "Area" can be defined as a function from a collection M of special kind of plane figures (termed measurable sets) to the set of real numbers which satisfies the following properties:

  • For all S in M, a(S) ≥ 0.
  • If S and T are in M then so are ST and ST, and also a(ST) = a(S) + a(T) − a(ST).
  • If S and T are in M with ST then TS is in M and a(TS) = a(T) − a(S).
  • If a set S is in M and S is congruent to T then T is also in M and a(S) = a(T).
  • Every rectangle R is in M. If the rectangle has length h and breadth k then a(R) = hk.
  • Let Q be a set enclosed between two step regions S and T. A step region is formed from a finite union of adjacent rectangles resting on a common base, i.e. SQT. If there is a unique number c such that a(S) ≤ c ≤ a(T) for all such step regions S and T, then a(Q) = c.

It can be proved that such an area function actually exists.[10]

Units

A square made of PVC pipe on grass
A square metre quadrat made of PVC pipe.

Every unit of length has a corresponding unit of area, namely the area of a square with the given side length. Thus areas can be measured in square metres (m2), square centimetres (cm2), square millimetres (mm2), square kilometres (km2), square feet (ft2), square yards (yd2), square miles (mi2), and so forth.[11] Algebraically, these units can be thought of as the squares of the corresponding length units.

The SI unit of area is the square metre, which is considered an SI derived unit.[3]

Conversions

A diagram showing the conversion factor between different areas
Although there are 10 mm in 1 cm, there are 100 mm2 in 1 cm2.

The conversion between two square units is the square of the conversion between the corresponding length units. For example, since

1 foot = 12 inches,

the relationship between square feet and square inches is

1 square foot = 144 square inches,

where 144 = 122 = 12 × 12. Similarly:

  • 1 square kilometer = 1,000,000 square meters
  • 1 square meter = 10,000 square centimetres = 1,000,000 square millimetres
  • 1 square centimetre = 100 square millimetres
  • 1 square yard = 9 square feet
  • 1 square mile = 3,097,600 square yards = 27,878,400 square feet

In addition,

Other units

{{#invoke:see also|seealso}} There are several other common units for area. The "Are" was the original unit of area in the metric system, with;

  • 1 are = 100 square metres

Though the are has fallen out of use, the hectare is still commonly used to measure land:[11]

  • 1 hectare = 100 ares = 10,000 square metres = 0.01 square kilometres

Other uncommon metric units of area include the tetrad, the hectad, and the myriad.

The acre is also commonly used to measure land areas, where

  • 1 acre = 4,840 square yards = 43,560 square feet.

An acre is approximately 40% of a hectare.

On the atomic scale, area is measured in units of barns, such that:[11]

  • 1 barn = 10−28 square meters.

The barn is commonly used in describing the cross sectional area of interaction in nuclear physics.[11]

In India,

  • 20 Dhurki = 1 Dhur
  • 20 Dhur = 1 Khatha
  • 20 Khata = 1 Bigha
  • 32 Khata = 1 Acre

History

Circle area

In the fifth century BCE, Hippocrates of Chios was the first to show that the area of a disk (the region enclosed by a circle) is proportional to the square of its diameter, as part of his quadrature of the lune of Hippocrates,[12] but did not identify the constant of proportionality. Eudoxus of Cnidus, also in the fifth century BCE, also found that the area of a disk is proportional to its radius squared.[13]

Subsequently, Book I of Euclid's Elements dealt with equality of areas between two-dimensional figures. The mathematician Archimedes used the tools of Euclidean geometry to show that the area inside a circle is equal to that of a right triangle whose base has the length of the circle's circumference and whose height equals the circle's radius, in his book Measurement of a Circle. (The circumference is 2πr, and the area of a triangle is half the base times the height, yielding the area πr2 for the disk.) Archimedes approximated the value of π (and hence the area of a unit-radius circle) with his doubling method, in which he inscribed a regular triangle in a circle and noted its area, then doubled the number of sides to give a regular hexagon, then repeatedly doubled the number of sides as the polygon's area got closer and closer to that of the circle (and did the same with circumscribed polygons).

Swiss scientist Johann Heinrich Lambert in 1761 proved that π, the ratio of a circle's area to its squared radius, is irrational, meaning it is not equal to the quotient of any two whole numbers.[14] French mathematician Adrien-Marie Legendre proved in 1794 that π2 is also irrational. In 1882, German mathematician Ferdinand von Lindemann proved that π is transcendental (not the solution of any polynomial equation with rational coefficients), confirming a conjecture made by both Legendre and Euler.[14]:p. 196

Triangle area

Heron (or Hero) of Alexandria found what is known as Heron's formula for the area of a triangle in terms of its sides, and a proof can be found in his book, Metrica, written around 60 CE. It has been suggested that Archimedes knew the formula over two centuries earlier,[15] and since Metrica is a collection of the mathematical knowledge available in the ancient world, it is possible that the formula predates the reference given in that work.[16]

In 499 Aryabhata, a great mathematician-astronomer from the classical age of Indian mathematics and Indian astronomy, expressed the area of a triangle as one-half the base times the height in the Aryabhatiya (section 2.6).

A formula equivalent to Heron's was discovered by the Chinese independently of the Greeks. It was published in 1247 in Shushu Jiuzhang (“Mathematical Treatise in Nine Sections”), written by Qin Jiushao.

Quadrilateral area

In the 600s CE, Brahmagupta developed a formula, now known as Brahmagupta's formula, for the area of a cyclic quadrilateral (a quadrilateral inscribed in a circle) in terms of its sides. In 1842 the German mathematicians Carl Anton Bretschneider and Karl Georg Christian von Staudt independently found a formula, known as Bretschneider's formula, for the area of any quadrilateral.

General polygon area

The development of Cartesian coordinates by René Descartes in the 1600s allowed the development of the surveyor's formula for the area of any polygon with known vertex locations by Gauss in the 1800s.

Areas determined using calculus

The development of integral calculus in the late 1600s provided tools that could subsequently be used for computing more complicated areas, such as the area of an ellipse and the surface areas of various curved three-dimensional objects.

Area formulas

Polygon formulas

{{#invoke:main|main}}

For a non-self-intersecting (simple) polygon, the Cartesian coordinates (i=0, 1, ..., n-1) of whose n vertices are known, the area is given by the surveyor's formula:[17]

where when i=n-1, then i+1 is expressed as modulus n and so refers to 0.

Rectangles

A rectangle with length and width labelled
The area of this rectangle is Template:Mvar.

The most basic area formula is the formula for the area of a rectangle. Given a rectangle with length Template:Mvar and width Template:Mvar, the formula for the area is:[2]

Template:Bigmath (rectangle)

That is, the area of the rectangle is the length multiplied by the width. As a special case, as l = w in the case of a square, the area of a square with side length Template:Mvar is given by the formula:[1][2]

Template:Bigmath (square)

The formula for the area of a rectangle follows directly from the basic properties of area, and is sometimes taken as a definition or axiom. On the other hand, if geometry is developed before arithmetic, this formula can be used to define multiplication of real numbers.

A diagram showing how a parallelogram can be re-arranged into the shape of a rectangle
Equal area figures.

Dissection, parallelograms, and triangles

{{#invoke:main|main}}

Most other simple formulas for area follow from the method of dissection. This involves cutting a shape into pieces, whose areas must sum to the area of the original shape.

For an example, any parallelogram can be subdivided into a trapezoid and a right triangle, as shown in figure to the left. If the triangle is moved to the other side of the trapezoid, then the resulting figure is a rectangle. It follows that the area of the parallelogram is the same as the area of the rectangle:[2]

Template:Bigmath  (parallelogram).
A parallelogram split into two equal triangles
Two equal triangles.

However, the same parallelogram can also be cut along a diagonal into two congruent triangles, as shown in the figure to the right. It follows that the area of each triangle is half the area of the parallelogram:[2]

 (triangle).

Similar arguments can be used to find area formulas for the trapezoid[18] as well as more complicated polygons.[19]

Area of curved shapes

Circles

A circle divided into many sectors can be re-arranged roughly to form a parallelogram
A circle can be divided into sectors which rearrange to form an approximate parallelogram.

{{#invoke:main|main}} The formula for the area of a circle (more properly called area of a disk) is based on a similar method. Given a circle of radius r, it is possible to partition the circle into sectors, as shown in the figure to the right. Each sector is approximately triangular in shape, and the sectors can be rearranged to form and approximate parallelogram. The height of this parallelogram is r, and the width is half the circumference of the circle, or πr. Thus, the total area of the circle is r × πr, or πr2:[2]

Template:Bigmath  (circle).

Though the dissection used in this formula is only approximate, the error becomes smaller and smaller as the circle is partitioned into more and more sectors. The limit of the areas of the approximate parallelograms is exactly πr2, which is the area of the circle.[20]

This argument is actually a simple application of the ideas of calculus. In ancient times, the method of exhaustion was used in a similar way to find the area of the circle, and this method is now recognized as a precursor to integral calculus. Using modern methods, the area of a circle can be computed using a definite integral:

Ellipses

{{#invoke:main|main}} The formula for the area of an ellipse is related to the formula of a circle; for an ellipse with semi-major and semi-minor axes x and y the formula is:[2]

Surface area

{{#invoke:main|main}}

A blue sphere inside a cylinder of the same height and radius
Archimedes showed that the surface area and volume of a sphere is exactly 2/3 of the area and volume of the surrounding cylindrical surface.

Most basic formulas for surface area can be obtained by cutting surfaces and flattening them out. For example, if the side surface of a cylinder (or any prism) is cut lengthwise, the surface can be flattened out into a rectangle. Similarly, if a cut is made along the side of a cone, the side surface can be flattened out into a sector of a circle, and the resulting area computed.

The formula for the surface area of a sphere is more difficult to derive: because a sphere has nonzero Gaussian curvature, it cannot be flattened out. The formula for the surface area of a sphere was first obtained by Archimedes in his work On the Sphere and Cylinder. The formula is:[6]

Template:Bigmath  (sphere).

where r is the radius of the sphere. As with the formula for the area of a circle, any derivation of this formula inherently uses methods similar to calculus.

General formulas

Areas of 2-dimensional figures

Area in calculus

A diagram showing the area between a given curve and the x-axis
Integration can be thought of as measuring the area under a curve, defined by f(x), between two points (here a and b).
A diagram showing the area between two functions
The area between two graphs can be evaluated by calculating the difference between the integrals of the two functions
  • The area between a positive-valued curve and the horizontal axis, measured between two values a and b (b is defined as the larger of the two values) on the horizontal axis, is given by the integral from a to b of the function that represents the curve:[1]
where is the curve with the greater y-value.

(see Green's theorem) or the z-component of

Bounded area between two quadratic functions

To find the bounded area between two quadratic functions, we subtract one from the other to write the difference as

where f(x) is the quadratic upper bound and g(x) is the quadratic lower bound. Define the discriminant of f(x)-g(x) as

By simplifying the integral formula between the graphs of two functions (as given in the section above) and using Vieta's formula, we can obtain[22][23]

The above remains valid if one of the bounding functions is linear instead of quadratic.

Surface area of 3-dimensional figures

General formula for surface area

The general formula for the surface area of the graph of a continuously differentiable function where and is a region in the xy-plane with the smooth boundary:

An even more general formula for the area of the graph of a parametric surface in the vector form where is a continuously differentiable vector function of is:[7]

List of formulas

Additional common formulas for area:
Shape Formula Variables
Regular triangle (equilateral triangle) is the length of one side of the triangle.
Triangle[1] is half the perimeter, , and are the length of each side.
Triangle[2] and are any two sides, and is the angle between them.
Triangle[1] and are the base and altitude (measured perpendicular to the base), respectively.
Isosceles triangle is the length of one of the two equal sides and is the length of a different side.
Rhombus/Kite and are the lengths of the two diagonals of the rhombus or kite.
Parallelogram is the length of the base and is the perpendicular height.
Trapezoid and are the parallel sides and the distance (height) between the parallels.
Regular hexagon is the length of one side of the hexagon.
Regular octagon is the length of one side of the octagon.
Regular polygon is the side length and is the number of sides.
Regular polygon is the perimeter and is the number of sides.
Regular polygon is the radius of a circumscribed circle, is the radius of an inscribed circle, and is the number of sides.
Regular polygon is the number of sides, is the side length, is the apothem, or the radius of an inscribed circle in the polygon, and is the perimeter of the polygon.
Circle is the radius and the diameter.
Circular sector and are the radius and angle (in radians), respectively and is the length of the perimeter.
Ellipse[2] and are the semi-major and semi-minor axes, respectively.
Total surface area of a cylinder and are the radius and height, respectively.
Lateral surface area of a cylinder and are the radius and height, respectively.
Total surface area of a sphere[6] and are the radius and diameter, respectively.
Total surface area of a pyramid[6] is the base area, is the base perimeter and is the slant height.
Total surface area of a pyramid frustum[6] is the base area, is the base perimeter and is the slant height.
Square to circular area conversion is the area of the square in square units.
Circular to square area conversion is the area of the circle in circular units.

The above calculations show how to find the areas of many common shapes.

The areas of irregular polygons can be calculated using the "Surveyor's formula".[20]

Relation of area to perimeter

The isoperimetric inequality states that, for a closed curve of length L (so the region it encloses has perimeter L) and for area A of the region that it encloses,

and equality holds if and only if the curve is a circle. Thus a circle has the largest area of any closed figure with a given perimeter.

At the other extreme, a figure with given perimeter L could have an arbitrarily small area, as illustrated by a rhombus that is "tipped over" arbitrarily far so that two of its angles are arbitrarily close to 0° and the other two are arbitrarily close to 180°.

For a circle, the ratio of the area to the circumference (the term for the perimeter of a circle) equals half the radius r. This can be seen from the area formula πr2 and the circumference formula 2πr.

The area of a regular polygon is half its perimeter times the apothem (where the apothem is the distance from the center to the nearest point on any side).

Fractals

Doubling the edge lengths of a polygon multiplies its area by four, which is two (the ratio of the new to the old side length) raised to the power of two (the dimension of the space the polygon resides in). But if the one-dimensional lengths of a fractal drawn in two dimensions are all doubled, the spatial content of the fractal scales by a power of two that is not necessarily an integer. This power is called the fractal dimension of the fractal. [25]

Area bisectors

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There are an infinitude of lines that bisect the area of a triangle. Three of them are the medians of the triangle (which connect the sides' midpoints with the opposite vertices), and these are concurrent at the triangle's centroid; indeed, they are the only area bisectors that go through the centroid. Any line through a triangle that splits both the triangle's area and its perimeter in half goes through the triangle's incenter (the center of its incircle). There are either one, two, or three of these for any given triangle.

Any line through the midpoint of a parallelogram bisects the area.

All area bisectors of a circle or other ellipse go through the center, and any chords through the center bisect the area. In the case of a circle they are the diameters of the circle.

Optimization

Given a wire contour, the surface of least area spanning ("filling") it is a minimal surface. Familiar examples include soap bubbles.

The question of the filling area of the Riemannian circle remains open.[26]

The circle has the largest area of any two-dimensional object having the same perimeter.

A cyclic polygon (one inscribed in a circle) has the largest area of any polygon with a given number of sides of the same lengths.

A version of the isoperimetric inequality for triangles states that the triangle of greatest area among all those with a given perimeter is equilateral.[27]

The triangle of largest area of all those inscribed in a given circle is equilateral; and the triangle of smallest area of all those circumscribed around a given circle is equilateral.[28]

The ratio of the area of the incircle to the area of an equilateral triangle, , is larger than that of any non-equilateral triangle.[29]

The ratio of the area to the square of the perimeter of an equilateral triangle, is larger than that for any other triangle.[27]

See also

References

  1. 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 Template:Cite web
  2. 2.00 2.01 2.02 2.03 2.04 2.05 2.06 2.07 2.08 2.09 2.10 Template:Cite web
  3. 3.0 3.1 Bureau International des Poids et Mesures Resolution 12 of the 11th meeting of the CGPM (1960), retrieved 15 July 2012
  4. {{#invoke:citation/CS1|citation |CitationClass=book }}
  5. {{#invoke:citation/CS1|citation |CitationClass=book }}
  6. 6.0 6.1 6.2 6.3 6.4 6.5 Template:Cite web
  7. 7.0 7.1 do Carmo, Manfredo. Differential Geometry of Curves and Surfaces. Prentice-Hall, 1976. Page 98, ISBN 978-0-13-212589-5
  8. Walter Rudin, Real and Complex Analysis, McGraw-Hill, 1966, ISBN 0-07-100276-6.
  9. Gerald Folland, Real Analysis: modern techniques and their applications, John Wiley & Sons, Inc., 1999,Page 20,ISBN 0-471-31716-0
  10. {{#invoke:citation/CS1|citation |CitationClass=book }}
  11. 11.0 11.1 11.2 11.3 {{#invoke:Citation/CS1|citation |CitationClass=journal }} Chapter 5.
  12. {{#invoke:citation/CS1|citation |CitationClass=citation }}.
  13. {{#invoke:citation/CS1|citation |CitationClass=book }}
  14. 14.0 14.1 {{#invoke:citation/CS1|citation |CitationClass=book }} English translation by Catriona and David Lischka.
  15. {{#invoke:citation/CS1|citation |CitationClass=book }}
  16. Weisstein, Eric W., "Heron's Formula", MathWorld.
  17. Template:Cite web
  18. {{#invoke:citation/CS1|citation |CitationClass=citation }}.
  19. {{#invoke:citation/CS1|citation |CitationClass=citation }}.
  20. 20.0 20.1 {{#invoke:Citation/CS1|citation |CitationClass=journal }}
  21. 21.0 21.1 {{#invoke:Citation/CS1|citation |CitationClass=journal }}
  22. Matematika - Marthen Kanginan, Google Books.
  23. Get Success UN +SPMB Matematika - Juliah, Google Books.
  24. 24.0 24.1 24.2 Template:Cite web
  25. {{#invoke:citation/CS1|citation |CitationClass=book }}
  26. {{#invoke:citation/CS1|citation |CitationClass=citation }}.
  27. 27.0 27.1 Chakerian, G. D. "A Distorted View of Geometry." Ch. 7 in Mathematical Plums (R. Honsberger, editor). Washington, DC: Mathematical Association of America, 1979: 147.
  28. Dorrie, Heinrich, 100 Great Problems of Elementary Mathematics, Dover Publ., 1965: 379-380.
  29. Minda, D., and Phelps, S., "Triangles, ellipses, and cubic polynomials", American Mathematical Monthly 115, October 2008, 679-689: Theorem 4.1.

External links

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