Hello all I encountered the following problem in my textbook: Evaluate the sum: 1*2 + 2*3 + ... + n(n+1) The answer I know is: n(n+1)(n+2)/ 3. However I was wondering how they got that. Here is my solution: v(v+1)(v+2) (we are assuming that n is a rational function). Now I need to know what to subtract from v(v+1)(v+2). What do I do next? I know you have to subtract an expression, but what is that expression? After subtracting you have to sum from v = 0 to v = n. And we get the answer. Any help would be greatly appreciated, as it will help in many other problems of the same type. Thanks!