Pentagonal pyramid

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Pentagonal pyramid
Pentagonal pyramid.png
Type Johnson
J1J2J3
Faces 5 triangles
1 pentagon
Edges 10
Vertices 6
Vertex configuration 5(32.5)
(35)
Schläfli symbol ( ) ∨ {5}
Symmetry group C5v, [5], (*55)
Rotation group C5, [5]+, (55)
Dual polyhedron self
Properties convex
Net
Pentagonal pyramid flat.svg
3D model of a pentagonal pyramid J2 pentagonal pyramid.stl
3D model of a pentagonal pyramid

In geometry, a pentagonal pyramid is a pyramid with a pentagonal base upon which are erected five triangular faces that meet at a point (the apex). Like any pyramid, it is self-dual.

Contents

The regular pentagonal pyramid has a base that is a regular pentagon and lateral faces that are equilateral triangles. It is one of the Johnson solids (J2).

It can be seen as the "lid" of an icosahedron; the rest of the icosahedron forms a gyroelongated pentagonal pyramid, J11.

More generally an order-2 vertex-uniform pentagonal pyramid can be defined with a regular pentagonal base and 5 isosceles triangle sides of any height.

Cartesian coordinates

The pentagonal pyramid can be seen as the "lid" of a regular icosahedron; the rest of the icosahedron forms a gyroelongated pentagonal pyramid, J11. From the Cartesian coordinates of the icosahedron, Cartesian coordinates for a pentagonal pyramid with edge length 2 may be inferred as

where 𝜏 (sometimes written as φ) is the golden ratio. [1]

The height H, from the midpoint of the pentagonal face to the apex, of a pentagonal pyramid with edge length a may therefore be computed as:

[2]

Its surface area A can be computed as the area of the pentagonal base plus five times the area of one triangle:

[3] [2]

Its volume can be calculated as:

[3]

The pentagrammic star pyramid has the same vertex arrangement, but connected onto a pentagram base:

Pentagram pyramid.png
Regular pyramids
Digonal Triangular Square Pentagonal Hexagonal HeptagonalOctagonalEnneagonalDecagonal...
ImproperRegularEquilateralIsosceles
Biangular pyramid1.png Tetrahedron.svg Square pyramid.png Pentagonal pyramid.png Hexagonal pyramid.png Heptagonal pyramid1.png Octagonal pyramid1.png Enneagonal pyramid1.png Decagonal pyramid1.png
Spherical digonal pyramid.png Spherical trigonal pyramid.png Spherical square pyramid.png Spherical pentagonal pyramid.png Spherical hexagonal pyramid.png Spherical heptagonal pyramid.png Spherical octagonal pyramid.png Spherical enneagonal pyramid.png Spherical decagonal pyramid.png
Pentagonal frustum.svg
Pentagonal frustum is a pentagonal pyramid with its apex truncated
Icosahedron.png
The top of an icosahedron is a pentagonal pyramid

Example

Pentagonal pyramid (at Matemateca IME-USP) Piramide pentagonal.jpg
Pentagonal pyramid (at Matemateca IME-USP)

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References

  1. Weisstein, Eric W. "Icosahedral Group". mathworld.wolfram.com. Retrieved 2020-04-12.
  2. 1 2 Sapiña, R. "Area and volume of a pentagonal pyramid and Johnson solid J₂". Problemas y ecuaciones (in Spanish). ISSN   2659-9899 . Retrieved 2020-06-29.
  3. 1 2 Weisstein, Eric W. "Pentagonal Pyramid". mathworld.wolfram.com. Retrieved 2020-04-12.