If an equilateral pentagon be inscribed in a circle, the square on the side of the pentagon is equal to the squares on the side of the hexagon and on that of the decagon inscribed in the same circle.
Proof
This is the Pythagorean relation p2=h2+d2 in the inscribed
polygons of a unit circle. Proven via I.47 applied to the right
triangle formed by the centre, a pentagon-vertex, and a decagon-vertex.