Question 266167
A polygon has n sides and n vertices.
A diagonal is obtained by joining two vertices.
Total number = nC2 = n*(n-1)/2
But this includes the n sides also.
So the number of diagonals = n*(n-1)/2 -n
                           =[n*(n-1)-2n]/2
                           = n*(n-1-2)
                           = n(n-3)/2