Question 557916
the number of sides is the same as the number of corners (vertices)


diagonals are lines from corner-to-corner, that aren't sides


if there are N corners; you can draw lines from any corner, to N-1 other corners


but, the line from A to B is the same as the line from B to A
___ so the number of lines is {[N(N - 1)] / 2}


N of the lines are sides, so the number of diagonals is ___ {[N(N - 1)] / 2} - N ___ or ___ {[N(N - 3)] / 2}