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More precisely, it is the solid figure bounded by a base in a plane and by a surface (called the lateral surface) formed by the locus of all straight line segments joining the apex to the perimeter of the base, such that there is a circular cross section. The term "cone" sometimes refers just to the surface of this solid figure, or just to the lateral surface.
The axis of a cone is the straight line (if any), passing through the apex, about which the base has a rotational symmetry.
In common usage in elementary geometry, cones are assumed to be right circular, where right means that the axis passes through the centre of the base (suitably defined) at right angles to its plane, and circular means that the base is a circle. Contrasted with right cones are oblique cones, in which the axis does not pass perpendicularly through the centre of the base. In general, however, the base may be any shape that permits a circular cross section of the cone, and the apex may lie anywhere (though it is usually assumed that the base is bounded and therefore has finite area, and that the apex lies outside the plane of the base).
The more general conic solid also has an apex and lines connecting the apex to all the points on a planar base, which can be of any shape. If the result has a circular cross section, it is a cone; if it has a polygonal base, it is a pyramid.
In mathematical usage, the word "cone" is used also for an 'infinite cone', the union of a set of half-lines that start at a common apex point and go through a base. Observe that an infinite cone is not bounded by its base and extends to infinity. A 'doubly infinite cone', or 'double cone', is the union of a set of straight lines that pass through a common apex point and go through a base, therefore double infinite cones extend symmetrically on both sides of the apex.
The boundary of an infinite or doubly infinite cone is a conical surface, and the intersection of a plane with this surface is a conic section. For infinite cones, the word axis again usually refers to the axis of rotational symmetry (if any). Either half of a double cone on one side of the apex is called a 'nappe'.
The perimeter of the base of a cone is called the 'directrix', and each of the line segments between the directrix and apex is a 'generatrix' of the lateral surface. (For the connection between this sense of the term "directrix" and the directrix of a conic section, see Dandelin spheres.)
The volume and the surface area for a straight cone are described in the geometry section below.
The 'base radius' of a circular cone is the radius of its base; often this is simply called the radius of the cone. The aperture of a right circular cone is the maximum angle between two generatrix lines; if the generatrix makes an angle θ to the axis, the aperture is 2θ.
A cone with its apex cut off by a plane is called a "truncated cone"; if the truncation plane is parallel to the cone's base, it is called a frustum. An 'elliptical cone' is a cone with an elliptical base. A 'generalized cone' is the surface created by the set of lines passing through a vertex and every point on a boundary (also see visual hull).
The lateral surface area of a right circular cone is where is the radius of the circle at the bottom of the cone and is the lateral height of the cone (given by the Pythagorean theorem where is the height of the cone). The surface area of the bottom circle of a cone is the same as for any circle, . Thus the total surface area of a right circular cone is:
The volume of any conic solid is one third of the product of the area of the base and the height (the perpendicular distance from the base to the apex).
In modern mathematics, this formula can easily be computed using calculus – it is, up to scaling, the integral Without using calculus, the formula can be proven by comparing the cone to a pyramid and applying Cavalieri's principle – specifically, comparing the cone to a (vertically scaled) right square pyramid, which forms one third of a cube. This formula cannot be proven without using such infinitesimal arguments – unlike the 2-dimensional formulae for polyhedral area, though similar to the area of the circle – and hence admitted less rigorous proofs before the advent of calculus, with the ancient Greeks using the method of exhaustion. This is essentially the content of Hilbert's third problem – more precisely, not all polyhedral pyramids are scissors congruent (can be cut apart into finite pieces and rearranged into the other), and thus volume cannot be computed purely by using a decomposition argument.
The center of mass of a conic solid of uniform density lies one-quarter of the way from the center of the base to the vertex, on the straight line joining the two.
For a circular cone with radius R and height H, the formula for volume becomes
where r is the radius of the cone at height h measured from the apex:
For a right circular cone, the surface area is
The first term in the area formula, , is the area of the base, while the second term, , is the area of the lateral surface.
A right circular cone with height and aperture , whose axis is the coordinate axis and whose apex is the origin, is described parametrically as
where range over , , and , respectively.
In implicit form, the same solid is defined by the inequalities
More generally, a right circular cone with vertex at the origin, axis parallel to the vector , and aperture , is given by the implicit vector equation where
where , and denotes the dot product.
In projective geometry, a cylinder is simply a cone whose apex is at infinity. Intuitively, if one keeps the base fixed and takes the limit as the apex goes to infinity, one obtains a cylinder, the angle of the side increasing as arctan, in the limit forming a right angle.
This is useful in the definition of degenerate conics, which require considering the cylindrical conics.
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