Formula: # of Diagonals in a Polygon

Hi we are Sierra and Odera and we're going teach you about the formula for diagonals.The formula is [(N- 3)n}/2. An easier way to put it is (n-3)X n/2.Lets say you have a octagon. First you think a octagon has a 8 sides
Then you write out formula. Then you erase all the N's and make them "8's." (or how ever many sides your polygon has) Then you follow order of operations .(PEMDAS)