Four Visual Proofs of a Theorem about a Regular Nonagon
This Demonstration shows four visual proofs that , where , , and are the lengths of a side, the shortest diagonal, and the longest diagonal, respectively, of a regular nonagon. The interior angle of the regular nonagon is , so that angle 451 equals .
 Š. Arslanagić, Matematička čitanka 4, Sarajevo, 2012, pp. 63–72.