Paint the pyramid
Source: 1998 HK
January 5, 2012
geometry3D geometrypyramidcombinatorics proposedcombinatorics
Problem Statement
The underside of a pyramid is a convex nonagon , paint all the diagonals of the nonagon and all the ridges of the pyramid into white and black , prove : there exists a triangle ,the colour of its three sides are the same . ( PS:the sides of the nonagon is not painted. )