Hendecagonal tegum

The hendecagonal bipyramid or hendpy, also called a hendecagonal dipyramid, is a bipyramid with a hendecagon as the midsection, constructed as the dual of a hendecagonal prism. It has 22 isosceles triangles as faces, with 2 order–11 and 11 order–4 vertices. .

In the variant obtained as the dual of a uniform hendecagonal prism, the side edges are $$\frac{1}{2\sin^2\frac{\pi}{11}} ≈ 6.29935$$ times the length of the edges of the base hendecagon. Each face has apex angle $$\arccos\left(1-2\sin^4\frac{\pi}{11}\right) \approx 9.10508°$$ and base angles $$\arccos\left(\sin^2\frac{\pi}{11}\right) \approx 85.44746°$$. If the base hendecagon has edge length 1, its height is $$\frac{\cos\frac{\pi}{11}}{\sin^2\frac{\pi}{11}} ≈ 12.08837$$.