A cone is a three-dimensional geometric shape that tapers smoothly from a flat, round base to a point called the apex or vertex. More precisely, it is the solid figure bounded by a plane base and the surface (called the lateral surface) formed by the locus of all straight line segments joining the apex to the perimeter of the base. 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 lateral surface has a rotational symmetry.
In general, the base may be any shape, and the apex may lie anywhere (though it is often assumed that the base is bounded and has nonzero area, and that the apex lies outside the plane of the base). For example, a pyramid is technically a cone with a polygonal base. In common usage in elementary geometry, however, 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.
- 1 Other mathematical meanings
- 2 Further terminology
- 3 Geometry
- 4 Bicone
- 5 Derivation of cone formulae from pyramid formulae
- 6 See also
- 7 External links
Other mathematical meanings
In mathematical usage, the word "cone" is used also for an infinite cone, the union of any set of half-lines that start at a common apex point. This kind of cone does not have a bounding base, and extends to infinity. A doubly infinite cone, or double cone, is the union of any set of straight lines that pass through a common apex point, and therefore extends 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). 1/2 of a double cone 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 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 parallel to its base is called a truncated cone or 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 volume of any conic solid is 1/3 the area of the base times the height (the perpendicular distance from the base to the apex).
The center of mass of a conic solid of uniform density lies 1/4 of the way from the center of mass of the base to the vertex, on the straight line joining the two.
So the volume of a cone is the volume of a cylinder divided by 3.
Right circular cone
For a circular cone with radius r and height h, the formula for volume becomes
For a right circular cone, the surface area is
- where is the slant height.
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.
A bicone or dicone is a solid formed by joining an cone and its mirror image base-to-base.
the name of the bicones is not an external face but an internal one, existing on the primary symmetry plane which connects the two cone halves.
For a circular bicone with radius r and height h, the formula for volume becomes
Surface area of a bicone with diameter , slant height is:
Derivation of cone formulae from pyramid formulae
The lateral area of a cone may be derived by approximating it with an n-sided pyramid and taking the limit as n approaches infinity.
where n is the number of sides of the pyramid's base. The pyramid's lateral area is given by
Taking the limit as n approaches infinity, we obtain the lateral area of the cone:
Since R is constant, this can be rewritten as:
Each limit in this expression can be evaluated separately.
- This expression can be proved to be equal to . Let L be the length of an edge in the pyramid's polygonal base. The relation between L and the polygon's radius R is
- The perimeter of the polygon is given by:
- Therefore, the perimeter of a regular polygon is:
- Taking the limit as n approaches infinity, we obtain the perimeter of the circular base of the cone:
- But since the ratio of a circle's perimeter to its diameter 2R is equal to , we have:
- Since 2R is constant and non-zero, this expression can be divided by 2R both in the nominator and the denominator, giving:
- Since , we have:
- So, the original square root becomes
- This expression is equal to the hypotenuse of the right triangle whose height is the height of the cone, and whose base is the radius of the cone. Therefore, is equal to the length of the line from the apex of the cone to its circular edge.
By putting together the elements of the original equation, we obtain:
Therefore, the lateral area of a cone is given by
- Cone (topology)
- Pyramid (geometry)
- Conic section
- Ruled surface
- Ice cream cone