2010 PUMaC Combinatorics A6: diagonals of regular decagon
Source:
August 21, 2011
geometrycircumcircle
Problem Statement
All the diagonals of a regular decagon are drawn. A regular decagon satisfies the property that if three diagonals concur, then one of the three diagonals is a diameter of the circumcircle of the decagon. How many distinct intersection points of diagonals are in the interior of the decagon?