Decagon

The decagon, or dec, is a polygon with 10 sides. A regular decagon has equal sides and equal angles.

The combining prefix is da-, as in dadip.

The only non-compound stellation of the decagon is the decagram. The only other polygons with a single non-compound stellation are the pentagon, the octagon, and the dodecagon.

Vertex coordinates
Coordinates for a decagon of unit edge length, centered at the origin are all sign changes of:


 * (±1/2, ±$\sqrt{(5+√5)/2}$/2),
 * (±(3+$\sqrt{5}$)/4, ±$\sqrt{(5+2√5)}$),
 * (±(1+$\sqrt{(5+2√5)}$)/2, 0).

Dipentagon
A dipentagon is a variant decagon with pentagonal symmetry, formed as a truncated pentagon. When decagons appear as faces in higher polytopes, they usually have this symmetry. Its dual is the pentambus.