Pentagonal tegum

The pentagonal bipyramid, or pedpy, also called a pentagonal dipyramid, is one of the 92 Johnson solids (J13). It has 10 equilateral triangles as faces, with 2 order-5 and 5 order-4 vertices. It can be constructed by joining two pentagonal pyramids at their bases

It is one of three regular polygonal bipyramids to be CRF. The others are the regular octahedron (square bipyramid) and the triangular bipyramid.

Vertex coordinates
A pentagonal bipyramid of edge length 1 has the following vertices:


 * $$\left(±\frac{1}{2},\, -\sqrt{\frac{5+2\sqrt{5}}{20}},\,0\right),$$
 * $$\left(±\frac{1+\sqrt{5}}{4},\, \sqrt{\frac{5-\sqrt{5}}{40}},\,0\right),$$
 * $$\left(0,\, \sqrt{\frac{5+\sqrt{5}}{10}},\,0\right),$$
 * $$\left(0,\,0,\,±\sqrt{\frac{5-\sqrt5}{10}}\right).$$

Representations
A pentagonal bipyramid has the following Coxeter diagrams:


 * oxo5ooo&#xt
 * yo ox5oo&#zx (y = $$\sqrt{\frac{10-2\sqrt5}{5}}$$)

Variations
The pentagonal bipyramid can have the height of its pyramids varied while maintaining its full symmetry These variations generally have 10 isosceles triangles for faces.

One notable variations can be obtained as the dual of the uniform pentagonal prism, which can be represented by m2m5o. In this variant the side edges are $$\frac{5+\sqrt5}{5} ≈ 1.44721$$ times the length of the edges of the base pentagon, and all the dihedral angles are $$\arccos\left(-\sqrt{\frac{11+4\sqr5}{41}}\right) ≈ 119.10723°$$. Each face has apex angle $$\arccos\left(\frac{\sqrt2+\sqrt{10}}{8}\right) \approx 55.10590°$$ and base angles $$\arccos\left(\frac{5-\sqrt5}{8}\right) \approx 69.78820°$$.

A pentagonal bipyramid with base edges of length b and side edges of length l has volume given by $$\frac{\sqrt{25+10\sqrt5}}{6}b^2\sqrt{l^2-b^2\frac{5+\sqrt5}{10}}$$.

Related polyhedra
A pentagonal prism can be inserted between the halves of the pentagonal bipyramid to produce the elongated pentagonal bipyramid. if a pentagonal antiprism is inserted instead, the result is the gyroelongated pentagonal bipyramid, better known as the regular icosahedron.