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| [[Image:Neusis-trisection.svg|thumb|right|Angles may be trisected via a [[Neusis construction]], but this uses tools outside the Greek framework of an unmarked [[straightedge]] and a [[compass (drafting)|compass]].]]
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| '''Angle trisection''' is a classic problem of [[compass and straightedge constructions]] of ancient [[Greek mathematics]]. It concerns construction of an [[angle]] equal to one third of a given arbitrary angle, using only two tools: an unmarked [[straightedge]], and a [[compass (drafting)|compass]].
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| The problem as stated is generally [[Proof of impossibility|impossible]] to solve, as shown by [[Pierre Wantzel]] (1837). Wantzel's proof relies on ideas from the field of [[Galois theory]]—in particular, trisection of an angle corresponds to the solution of a certain [[cubic equation]], which is not possible using the given tools. Note that the fact that there is no way to trisect an angle ''in general'' with just a compass and a straightedge does not mean that there is ''no'' trisectible angle: for example, it is relatively straightforward to trisect a [[right angle]] (that is, to construct an angle of measure 30 degrees).
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| It is, however, possible to trisect an arbitrary angle, but using tools other than straightedge and compass. For example, [[neusis construction]], also known to ancient Greeks, involves simultaneous sliding and rotation of a marked straightedge, which cannot be achieved with the original tools. Other techniques were developed by mathematicians over centuries.
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| Because it is defined in simple terms, but complex to prove unsolvable, the problem of angle trisection is a frequent subject of [[pseudomathematics|pseudomathematical]] attempts at solution by naive enthusiasts. The "solutions" often involve mistaken interpretations of the rules, or are simply incorrect.<ref>{{cite web |url=http://www.uwgb.edu/dutchs/pseudosc/trisect.htm |title=Why Trisecting the Angle is Impossible |author=Steven Dutch |publisher=University of Wisconsin - Green Bay}}</ref>
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| ==Background and problem statement==
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| [[Image:Bisection construction.gif|thumb||[[Bisection]] of [[arbitrary]] [[angle]]s has long been solved.]]
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| Using only an unmarked [[straightedge]] and a [[compass (drafting)|compass]], [[Greek mathematics|Greek mathematicians]] found means to divide a [[Line (mathematics)|line]] into an arbitrary set of equal segments, to draw [[Parallel (geometry)|parallel]] lines, to [[Bisection#Angle_bisector|bisect angle]]s, to construct many [[polygon]]s, and to construct [[Square (geometry)|square]]s of equal or twice the area of a given polygon.
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| Three problems proved elusive, specifically, trisecting the angle, [[doubling the cube]], and [[squaring the circle]]. The problem of angle trisection reads:
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| Construct an [[angle]] equal to one-third of a given arbitrary angle (or divide it into three equal angles), using only two tools:
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| # an un-marked [[straightedge]] ''and''
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| # a [[compass (drafting)|compass]].
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| == Proof of impossibility ==
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| [[File:Lineale.jpg|thumb|right|[[Ruler]]s. The displayed ones are marked — an ideal [[straightedge]] is un-marked]]
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| [[File:Zirkel.jpg|thumb|right|[[compass (drafting)|compasses]]]]
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| The problem of constructing an angle of a given measure <math>\theta</math> is equivalent to constructing two segments such that the ratio of their length is <math>\cos\theta,</math> because one may pass from one solution to the other by a compass and straightedge construction. It follows that, given a segment that is sought as having a unit length,
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| the problem of angle trisection is equivalent to constructing a segment whose length is the root of a [[cubic polynomial]] — since by the [[triple-angle formula]], <math>\cos\theta=4\cos^3(\theta/3)-3\cos(\theta/3).</math>
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| This allows to reduce the original geometric problem to a purely algebraic problem.
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| One can show that every rational number is constructible and that every [[irrational number]] which is [[constructible number|constructible]] in one step from some given numbers is a root of a [[polynomial]] of degree 2 with coefficients in the [[field (mathematics)|field]] generated by these numbers. Therefore any number which is constructible by a series of steps is a root of a [[minimal polynomial (field theory)|minimal polynomial]] whose degree is a power of 2. Note also that <math>\pi/3</math> [[radian]]s (60 [[degree (angle)|degree]]s, written 60°) is [[equilateral triangle|constructible]]. We now show that it is impossible to construct a 20° angle; this implies that a 60° angle cannot be trisected, and thus that an arbitrary angle cannot be trisected.
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| Denote the set of [[rational numbers]] by '''Q'''. If 60° could be trisected, the degree of a minimal polynomial of {{math|cos(20°)}} over '''Q''' would be a power of two. Now let {{math|''y'' {{=}} cos(20°)}}.
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| Note that {{math|cos(60°)}}<math>= \cos(\pi/3) = 1/2</math>. Then by the triple-angle formula, <math>\cos(\pi/3)= 1/2 = 4y^{3} - 3y</math> and so <math>4y^{3} - 3y - 1/2 = 0</math>. Thus <math>8y^{3} - 6y - 1 = 0</math>, or equivalently <math>(2y)^{3} - 3(2y) - 1 = 0</math>. Now substitute <math>x = 2y</math>, so that <math>x^{3} - 3x - 1 = 0 </math>. Let <math>p(x) = x^{3} - 3x - 1</math>.
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| The minimal polynomial for ''x'' (hence {{math|cos(20°)}}) is a factor of <math>p(x)</math>. Because <math>p(x)</math> is degree 3, if it is reducible over by '''Q''' then it has a [[rational root]]. By the [[rational root theorem]], this root must be 1 or −1, but both are clearly not roots. Therefore <math>p(x)</math> is [[irreducible polynomial|irreducible]] over by '''Q''', and the minimal polynomial for {{math|cos(20°)}} is of degree 3.
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| So an angle of 60° = (1/3)π [[radian]]s cannot be trisected.
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| Many people (who presumably are unaware of the above result, misunderstand it, or incorrectly reject it) have proposed methods of trisecting the general angle. Some of these methods provide reasonable approximations; others (some of which are mentioned below) involve tools not permitted in the classical problem. The mathematician [[Underwood Dudley]] has detailed some of these failed attempts in his book ''The Trisectors''.<ref>Dudley, Underwood, ''The Trisectors'', Mathematical Association of America, 1994.</ref>
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| == Angles which can be trisected ==
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| However, some angles can be trisected. For example, for any [[constructible number|constructible]] angle <math>\theta</math>, the angle <math>3\theta</math> can be trivially trisected by ignoring the given angle and directly constructing an angle of measure <math>\theta</math>. There are angles which are not constructible, but are trisectible. For example, <math>3\pi/7</math> is such an angle: five copies of <math>3\pi/7</math> combine to make an angle of measure <math>15\pi/7</math>, which is a full circle plus the desired <math>\pi/7</math>. More generally, for a [[positive integer]] <math>N</math>, an angle of measure <math>2\pi/N</math> is trisectible if and only if <math>3</math> does not divide <math>N</math>;<ref>McLean, K. Robin, "Trisecting angles with ruler and compasses", ''Mathematical Gazette'' 92, July 2008, 320–323. See also Feedback on this article in vol. 93, March 2009, p. 156.</ref> if <math>N>2</math> is a [[prime number]], this angle is constructible if and only if <math>N</math> is a [[Fermat prime]].
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| ===One general theorem===
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| Again, denote the [[rational numbers]] as '''Q''':
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| [[Theorem]]: The angle <math>\theta</math> may be trisected [[if and only if]] <math>q(t) = 4t^{3}-3t-\cos(\theta)</math> is reducible over the [[field extension]] '''Q'''<math>(\cos(\theta))</math>.
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| The [[Mathematical proof|proof]] is a relatively straightforward generalization of the proof given above that a 60-degree angle is not trisectible.<ref name=Stewart>{{cite book | last = Stewart | first = Ian | authorlink = Ian Stewart (mathematician) | title = ''[[Galois Theory]]'' | publisher = Chapman and Hall Mathematics | year = 1989 | pages = g. 58 | doi = | isbn = 0-412-34550-1 }}</ref>
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| ==Trisection using other methods==
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| The general problem of angle trisection is solvable, but using additional tools, and thus going outside of the original Greek framework of compass and straightedge.
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| ===By infinite repetition of bisection===
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| Trisection can be achieved by infinite repetition of the compass and straightedge method for bisecting an angle. The geometric series 1/3 = 1/4+1/16+1/64+1/256+... or 1/3 = 1/2-1/4+1/8-1/16+... can be used as a basis for the bisections. This method is considered to be breaking the rules for compass and straightedge construction as it involves an infinite number of steps. However, an approximation to any degree of accuracy can be obtained in a finite number of steps.<ref>{{cite web| title=Trisection of an Angle | author=Jim Loy | date=1997, 2003 | url=http://www.jimloy.com/geometry/trisect.htm | accessdate=30 March 2012}}</ref>
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| ===Using origami===
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| {{main|Mathematics of origami#Trisecting an angle}}
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| Trisection, like many constructions impossible by ruler and compass, can easily be accomplished by the more powerful operations of paper folding, or [[origami]]. [[Huzita's axioms]] (types of folding operations) can construct cubic extensions (cube roots) of given lengths, whereas ruler-and-compass can construct only quadratic extensions (square roots).
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| ===With an auxiliary curve===
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| There are certain curves called [[trisectrix|trisectrices]] which, if drawn on the plane using other methods, can be used to trisect arbitrary angles.<ref>[http://www.jimloy.com/geometry/trisect.htm#curves Trisection of an Angle<!-- Bot generated title -->]</ref>
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| ===With a marked ruler===
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| [[Image:Three facts for trisecting angles.svg|right|355px|thumb|basic triangle's angle properties]]
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| Another means to trisect an arbitrary angle by a "small" step outside the Greek framework is via a ruler with two marks a set distance apart. The next construction is originally due to [[Archimedes]], called a ''[[Neusis construction]]'', i.e., that uses tools other than an ''un-marked'' straightedge. The diagrams we use show this construction for an acute angle, but it indeed works for any angle up to 180 degrees.
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| This requires three facts from geometry (at right):
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| # Any full set of angles on a straight line add to 180°,
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| # The sum of angles of any triangle is 180°, ''and'',
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| # Any two equal sides of an [[isosceles triangle]] will [[Pons asinorum|meet the third in the same angle]].
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| {{Clear}}
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| [[File:Trisecting angles three.svg|thumb|355px|Trisection of the angle using marked ruler]]
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| Let ''l'' be the horizontal line in the diagram on the right. Angle ''a'' (left of point ''B'') is the subject of trisection. First, a point ''A'' is drawn at an angle's [[ray (geometry)|ray]], one unit apart from ''B''. A circle of [[radius]] ''AB'' is drawn. Then, the markedness of the ruler comes into play: one mark of the ruler is placed at ''A'' and the other at ''B''. While keeping the ruler (but not the mark) touching ''A'', the ruler is slid and rotated until one mark is on the circle and the other is on the line ''l''. The mark on the circle is labeled ''C'' and the mark on the line is labeled ''D''. This ensures that ''CD = AB''. A radius ''BC'' is drawn to make it obvious that line segments ''AB'', ''BC'', and ''CD'' all have equal length. Now, Triangles ''ABC'' and ''BCD'' are [[isosceles triangle|isosceles]], thus (by Fact 3 above) each has two equal angles.
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| [[Hypothesis]]: Given ''AD'' is a straight line, and ''AB'', ''BC'', and ''CD'' are all equal length,
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| [[logical consequence|Conclusion]]: angle <math> b = (1/3) a </math>.
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| [[Mathematical proof|Proof]]:
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| # From Fact 1) above, <math> e + c = 180</math>°.
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| # Looking at triangle ''BCD'', from Fact 2) <math> e + 2b = 180</math>°.
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| # From the last two equations, <math> c = 2b</math>.
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| # From Fact 2), <math> d + 2c = 180</math>°, thus <math> d = 180</math>°<math> - 2c </math>, so from last, <math> d = 180</math>°<math> - 4b</math>.
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| # From Fact 1) above, <math> a + d + b = 180</math>°, thus <math> a + (180</math>°<math> - 4b) + b = 180</math>°.
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| Clearing, <math> a - 3b = 0 </math>, or <math> a = 3b </math>, and the [[theorem]] is [[Q.E.D.|proved]].
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| Again, this construction stepped outside the [[Greek mathematics|framework]] of [[compass and straightedge constructions|allowed constructions]] by using a marked straightedge.
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| ===With a string===
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| Thomas Hutcheson published an article in the [[Mathematics Teacher]]<ref>{{cite journal|journal=Mathematics Teacher|volume=94 |issue=5 |date=May 2001 |pages=400–405 |last=Hutcheson |first=Thomas W. |title=Dividing Any Angle into Any Number of Equal Parts}}</ref> that used a string instead of a compass and straight edge. A string can be used as either a straight edge (by stretching it) or a compass (by fixing one point and identifying another), but can also wrap around a cylinder, the key to Hutcheson's solution.
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| Hutcheson constructed a cylinder from the angle to be trisected by drawing an arc across the angle, completing it as a circle, and constructing from that circle a cylinder on which a, say, equilateral triangle was inscribed (a 360-degree angle divided in three). This was then "mapped" onto the angle to be trisected, with a simple proof of similar triangles.
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| ===With a "tomahawk"===
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| [[Image:TomahawkTrisecting.svg|thumb|right|A tomahawk trisecting an angle. The handle forms one trisector and the blue line shown forms the other.]]
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| A "[[Tomahawk (geometric shape)|tomahawk]]" is a geometric shape consisting of a semicircle and two orthogonal line segments, such that the length of the shorter segment is equal to the circle radius. Trisection is executed by leaning the end of the tomahawk's shorter segment on one ray, the circle's edge on the other, so that the "handle" (longer segment) crosses the angle's vertex; the trisection line runs between the vertex and the center of the semicircle.
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| Note that while a tomahawk is constructible with compass and straightedge, it is not generally possible to construct a tomahawk in any desired position. Thus, the above construction does not contradict the nontrisectibility of angles with ruler and compass alone.
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| The tomahawk produces the same geometric effect as the paper-folding method: the distance between circle center and the tip of the shorter segment is twice the distance of the radius, which is guaranteed to contact the angle. It is also equivalent to the use of an architects L-Ruler ([[Steel_square#Carpenter's_square | Carpenter's Square]]).
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| ===With interconnected compasses===
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| An angle can be trisected with a device that is essentially a four-pronged version of a compass, with linkages between the prongs designed to keep the three angles between adjacent prongs equal.<ref>Isaac, Rufus, "Two mathematical papers without words", ''[[Mathematics Magazine]]'' 48, 1975, p. 198. Reprinted in ''Mathematics Magazine'' 78, April 2005, p. 111.</ref>
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| ==See also==
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| *[[Bisection]]
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| *[[Constructible number]]
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| *[[Constructible polygon]]
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| *[[Euclidean geometry]]
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| *[[History of geometry]]
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| *[[Intercept theorem]]
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| *[[List of geometry topics]]
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| *[[Morley's trisector theorem]]
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| *[[Quadratrix]]
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| *[[Trisectrix]]
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| ==References==
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| {{Reflist|colwidth=30em}}
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| ==Additional references==
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| *Courant, Richard, Herbert Robbins, Ian Stewart, ''What is mathematics?: an elementary approach to ideas and methods'', Oxford University Press US, 1996. ISBN 978-0-19-510519-3.
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| *Raghavendran, K. "Tripedal dividers of angles", ''Proceedings of Third International Measurement Conference (IMEKOIII)'', Stockholm, Sept. 1964.
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| ==External links==
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| *[http://mathworld.wolfram.com/AngleTrisection.html MathWorld site]
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| *[http://mathworld.wolfram.com/GeometricProblemsofAntiquity.html Geometric problems of antiquity, including angle trisection]
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| *[http://www-history.mcs.st-andrews.ac.uk/HistTopics/Trisecting_an_angle.html Some history]
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| *[http://www.uwgb.edu/dutchs/PSEUDOSC/trisect.HTM One link of marked ruler construction]
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| *[http://www.cut-the-knot.org/pythagoras/archi.shtml Another, mentioning Archimedes]
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| *[http://www.jimloy.com/geometry/trisect.htm A long article with many approximations & means going outside the Greek framework]
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| *[http://www.geom.uiuc.edu/docs/forum/angtri/ Geometry site]
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| ===Other means of trisection===
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| *[http://trisectlimacon.webs.com/ Trisecting via] ([http://www.webcitation.org/5knI8nq2l Archived] 2009-10-25) the ''[[limacon]] of [[Blaise Pascal|Pascal]]''; see also ''[[Trisectrix]]''
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| *[http://www.uwgb.edu/dutchs/PSEUDOSC/trisect.HTM Trisecting via] an ''[[Archimedean Spiral]]''
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| *[http://xahlee.org/SpecialPlaneCurves_dir/ConchoidOfNicomedes_dir/conchoidOfNicomedes.html Trisecting via] the ''[[Conchoid (mathematics)|Conchoid]] of [[Nicomedes (mathematician)|Nicomedes]]''
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| *[http://www.sciencenews.org/articles/20070602/mathtrek.asp sciencenews.org site] on using [[origami]]
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| *[http://www.song-of-songs.net/Star-of-David-Flower-of-Life.html Hyperbolic trisection and the spectrum of regular polygons]
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| {{Greek mathematics}}
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| [[Category:Euclidean plane geometry|*]]
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| [[Category:Mathematical problems]]
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| [[Category:Articles containing proofs]]
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| [[Category:History of geometry]]
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| [[Category:Compass and straightedge constructions]]
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