1. The problem statement, all variables and given/known data If P r=(n-r)(n-r+1)(n-r+2).....(n-r+p-1) Qr= r(r+1)(r+2)......(r+q-1) Find P1Q1+P2Q2+....... +Pn-1Qn-1 2. Relevant equations 3. The attempt at a solution I tried to bring the general term in the form of a coefficient of x in the binomial expansion. But it does not simplify to that form. can anyone give me a better way to approach the problem?