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| [[Image:DoubleCone.png|thumb|250px|right|A circular conical surface]]
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| In [[geometry]], a ('''general''') '''conical surface''' is the unbounded [[surface]] formed by the union of all the straight [[line (mathematics)|lines]] that pass through a fixed point — the ''apex'' or ''vertex'' — and any point of some fixed [[space curve]] — the ''directrix'' — that does not contain the apex. Each of those lines is called a ''generatrix'' of the surface.
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| Every conic surface is [[ruled surface|ruled]] and [[developable surface|developable]]. In general, a conical surface consists of two congruent unbounded halves joined by the apex. Each half is called a '''nappe''', and is the union of all the [[Line (mathematics)#Ray|ray]]s that start at the apex and pass through a point of some fixed space curve. (In some cases, however, the two nappes may intersect, or even coincide with the full surface.) Sometimes the term "conical surface" is used to mean just one nappe.
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| If the directrix is a circle <math>C</math>, and the apex is located on the circle's ''axis'' (the line that contains the center of <math>C</math> and is perpendicular to its plane), one obtains the ''right circular conical surface''. This special case is often called a ''[[cone (geometry)|cone]]'', because it is one of the two distinct surfaces that bound the [[geometric solid]] of that name. This geometric object can also be described as the set of all points swept by a line that intercepts the axis and [[rotation|rotates]] around it; or the union of all lines that intersect the axis at a fixed point <math>p</math> and at a fixed angle <math>\theta</math>. The ''aperture'' of the cone is the angle <math>2 \theta</math>.
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| More generally, when the directrix <math>C</math> is an [[ellipse]], or any [[conic section]], and the apex is an arbitrary point not on the plane of <math>C</math>, one obtains a ''conical quadric'', which is a special case of a [[quadric|quadric surface]].
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| A [[cylindrical surface]] can be viewed as a [[limiting case]] of a conical surface whose apex is moved off to infinity in a particular direction. Indeed, in [[projective geometry]] a cylindrical surface is just a special case of a conical surface.
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| ==Equations==
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| A conical surface <math>S</math> can be described [[parametrization|parametrically]] as
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| :<math>S(t,u) = v + u q(t)</math>, | |
| where <math>v</math> is the apex and <math>q</math> is the directrix.
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| A right circular conical surface of aperture <math>2\theta</math>, whose axis is the <math>z</math> coordinate axis, and whose apex is the origin, it is described parametrically as
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| :<math>S(t,u) = (u \cos\theta \cos t, u \cos\theta \sin t, u \sin\theta)</math> | |
| where <math>t</math> and <math>u</math> range over <math>[0,2\pi)</math> and <math>(-\infty,+\infty)</math>, respectively. In [[implicit geometric model|implicit]] form, the same surface is described by <math>S(x,y,z) = 0</math> where
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| :<math>S(x,y,z) = (x^2 + y^2)(\cos\theta)^2 - z^2 (\sin \theta)^2.</math>
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| More generally, a right circular conical surface with apex at the origin, axis parallel to the vector <math>\mathbf{d}</math>, and aperture <math>2\theta</math>, is given by the implicit [[vector calculus|vector]] equation <math>S(\mathbf{x}) = 0</math> where
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| :<math>S(\mathbf{x}) = (\mathbf{x} \cdot \mathbf{d})^2 - (\mathbf{d} \cdot \mathbf{d}) (\mathbf{x} \cdot \mathbf{x}) (\cos \theta)^2</math> | |
| or
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| :<math>S(\mathbf{x}) = \mathbf{x} \cdot \mathbf{d} - |\mathbf{d}| |\mathbf{x}| \cos \theta</math>
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| where <math>\mathbf{x}=(x,y,z)</math>, and <math>\mathbf{x} \cdot \mathbf{d}</math> denotes the [[dot product]].
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| In three coordinates, x, y and z, the general equation for a cone with apex at origin is a homogenous equation of degree 2 given by
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| :<math>S(x, y, z) = ax^2+by^2+cz^2+2uxy+2vyz+2wzx=0</math>
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| ==See also==
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| *[[Conic section]]
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| *[[Developable surface]]
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| *[[Quadric]]
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| *[[Ruled surface]]
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| {{DEFAULTSORT:Conical Surface}}
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| [[Category:Euclidean solid geometry]]
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| [[Category:Surfaces]]
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| [[Category:Algebraic surfaces]]
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| [[Category:Quadrics]]
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