Find the sum of the first n terms of the series whose nth term is `n(n+3)`.

Explanation

Given that `a_n=n(n+3)=n^2+3 n`
Thus, the sum to n terms is given by

`S_n=sum_k=1^n a_k=sum_k=1^n k^2+3 sum_k=1^n dotk^=n(n+1)(2 n+1)/6+3 n(n+1)/2=n(n+1)(n+5)/3`