Let us have a look at this series:
nm + (n-1)(m-1) + (n-2)(m-2) + (n-3)(m-3) + ...... + (until either n==1 or m==1).
Let, n=5, m=3. So we need to find the summation of
5.3 + 4.2 + 3.1
Well, what I need is a closed form. And how should I find out a closed form of such kind of series?
Thanks in advance! :)