What is the formula for the sum?

Question

1*2 + 2*3 + 3*4 + 4*5 .....+n*n+1

what i the sum of N?

Answer 1

Have you written this down correctly - because I think the sum of the series you have defined would tend to infinity. Do you mean Sum of 1/(n * (n + 1)) for all values of n from 1 to infinity?

Answer 2

2*C(n,3)

with
C(n,k) = n!/k!(n-k)!

n! = n * n-1 * n-2 ... * 1