Points connected by red and blue line segments
Source: Canadian Mathematical Olympiad 1976 Problem 8
October 23, 2008
combinatorics proposedcombinatorics
Problem Statement
Each of the 36 line segments joining 9 distinct points on a circle is coloured either red or blue. Suppose that each triangle determined by 3 of the 9 points contains at least one red side. Prove that there are four points such that the 6 segments connecting them are all red.