Find the closed form sum of 1*2*3 + 2*3*4 + 3*4*5 + ... + n(n + 1)(n + 2).
sum_(k=1)^n k (k + 1) (k + 2) = 1/4 n (n + 1) (n + 2) (n + 3) - this is the closed form.
