Question

I can't find the formula for the sides of a polygon vs. the total diagonals that it has! ex: 3 sided polygon, 0 diagonals, 4 sided, 2 diags, 5 s, 5 d, 6 s, 9 d, 7 s, 14 d 8 s, 20d, etc.
someone please tell me the formula

http://mathforum.org/library/drmath/view/54874.html