Pyramid (geometry)

This article is about the polyhedron pyramid (a 3-dimensional shape). For other versions including architectural pyramids, see Pyramid (disambiguation).

Set of pyramids
Schläfli symbol	{ } v {n}
Faces	n triangles, 1 n-gon
Edges	2n
Vertices	n + 1
Symmetry group	Cnv, [1,n], (*nn), order 2n
Rotation group	Cn, [1,n+], (nn), order n
Dual polyhedron	Self-dual
Properties	convex

The 1-skeleton of pyramid is a wheel graph

In geometry, a pyramid is a polyhedron formed by connecting a polygonal base and a point, called the apex. Each base edge and apex form a triangle. It is a conic solid with polygonal base.

A pyramid with an n-sided base will have n + 1 vertices, n + 1 faces, and 2n edges. All pyramids are self-dual.

When unspecified, the base is usually assumed to be square.

If the base is a regular polygon and the apex is above the center of the polygon, an n-gonal pyramid will have C_nv symmetry.

If all edges of a pyramid (or any convex polyhedron) are tangent to a sphere so that the average position of the tangential points are at the center of the sphere, then the pyramid is said to be canonical, and it forms half of the dual polyhedron of a cube.

Pyramids are a subclass of the prismatoids.

Pyramids with regular polygon faces

The trigonal or triangular pyramid with all equilateral triangles faces becomes the regular tetrahedron, one of the Platonic solids. A lower symmetry case of the triangular pyramid is C_3v which has an equilateral triangle base, and 3 identical isosceles triangle sides. The square and pentagonal pyramids can also be composed of regular convex polygons, in which case they are Johnson solids.

Pyramids

Tetrahedron	Square pyramid	Pentagonal pyramid	Hexagonal pyramid

Star pyramids

Pyramids with regular star polygon bases are called star pyramids. For example, the pentagrammic pyramid has a pentagram base and 5 intersecting triangle sides.

Volume

See also: Cone (geometry) – Volume

The volume of a pyramid (also any cone) is $\scriptstyle{V=} \tfrac{1}{3}\scriptstyle{bh}$ where b is the area of the base and h the height from the base to the apex. This works for any polygon, regular or non-regular, and any location of the apex, provided that h is measured as the perpendicular distance from the plane which contains the base. In 499 AD Aryabhata, a mathematician-astronomer from the classical age of Indian mathematics and Indian astronomy, used this method in the Aryabhatiya (section 2.6).

The formula can be formally proved using calculus: By similarity, the linear dimensions of a cross section parallel to the base increase linearly from the apex to the base. The scaling factor (proportionality factor) is $\scriptstyle{1 -} \tfrac{y}{h}$, or $\scriptstyle\tfrac{h - y}{h}$, where h is the height and y is the perpendicular distance from the plane of the base to the cross-section. Since the area of any cross section is proportional to the square of the shape's scaling factor, the area of a cross section at height y is B×$\tfrac{(h - y)^2}{h^2}$, or since both b and h are constants $\tfrac{b}{h^2}$$\scriptstyle (h - y)^2$. The volume is given by the integral

$\frac{b}{h^2} \int_0^h (h-y)^2 \, dy = \frac{-b}{3h^2} (h-y)^3 \bigg|_0^h = \tfrac{1}{3}bh.$

The same equation, $\scriptstyle{V=} \tfrac{1}{3}\scriptstyle{bh}$, also holds for cones with any base. This can be proven by an argument similar to the one above; see volume of a cone.

For example, the volume of a pyramid whose base is an n-sided regular polygon with side length s and whose height is h is:

$V = \frac{n}{12}hs^2 \cot\frac{\pi}{n}.$

Surface area

The surface area of a pyramid is $\scriptstyle{A= B +} \tfrac{PL}{2}$ where B is the base area, P is the base perimeter and L is the slant height $\scriptstyle{L=} \sqrt{\scriptstyle{h^2+r^2}}$ where h is the pyramid altitude and r is the inradius of the base.

n-dimensional pyramids

A 2-dimensional pyramid is a triangle, formed by a base edge connected to a noncolinear point called an apex.

A 4-dimensional pyramid is called a polyhedral pyramid, constructed by a polyhedron in a 3-space hyperplane of 4-space with another point off that hyperplane.

Higher dimensional pyramids are constructed similarly.

The family of simplices represent pyramids in any dimension, increasing from triangle, tetrahedron, 5-cell, 5-simplex, ... A n-dimensional simplex has the minimum n+1 vertices, with all pairs of vertices connected by edges, all triples of vertices defining faces, all quadruples of points defining tetrahedral cells, etc.

See also

• Bipyramid
• Cone (geometry)
• Trigonal pyramid (chemistry)
• Frustum

External links

• Weisstein, Eric W., "Pyramid", MathWorld.
• Olshevsky, George, Pyramid at Glossary for Hyperspace.
• The Uniform Polyhedra