Hi

I'm not sure if it belongs in calculus but here it is anyway:

Find the sum of $\displaystyle \displaystyle\sum_{i=1}^{\infty}\frac{i^2}{a^i}$ for a constant $\displaystyle a$

Wolframalpha yielded $\displaystyle \frac{a(a+1)}{(a-1)^3}$ but I would like to know how to derive it by hand.