Question 68
A polygon having $n$ sides is arbitrarily decomposed in triangles having all the vertices among the vertices of the polygon. We paint in black the triangles that have two sides that are also sides of the polygon, in red if only one side of the triangle is side of the polygon and white those triangles that have in common with the polygon only vertices.
Prove that there are 2 more black triangles than white ones.
Prove that there are 2 more black triangles than white ones.