calculus - To evaluate $int_{0}^pi frac {sin^2 x}{a^2-2ab cos x + b^2}mathrm dx$?

How do we evaluate $\displaystyle\int_{0}^\pi \dfrac {\sin^2 x}{a^2-2ab \cos x + b^2}\mathrm dx$ ? I tried substitution and some other methods , but its not working ; please help .