maths and stat(combination)

There are nC2 ways to choose any 2 vertices. Out of these, n are to be excluded as they are the sides of the polygon.
The number of diagonals = nC2 – n
= n(n – 1)/2 – n
= n(n – 3)/2

2010-06-24 12:32:35 補充：
對角線要選擇其中2個頂點
但nC2是包括該POLYGON的邊.....所以要減走多邊形的邊才是答案