20C3 is the total number of triangles, including those that use the polygon's sides. You need to subtract such triangles. For this it is convenient to count separately the number of triangles that use exactly one or exactly two of polygon's sides.
20C3 is the total number of triangles, including those that use the polygon's sides. You need to subtract such triangles. For this it is convenient to count separately the number of triangles that use exactly one or exactly two of polygon's sides.