# Math Help - integrl question..

1. ## integrl question..

$\int_{-\pi}^{\pi}\sin x \sin 2x$

i tried to solve it by parts
but both sides change to cosine

??

2. Originally Posted by transgalactic
$\int_{-\pi}^{\pi}\sin x \sin 2x$
Use this identity : $\sin p \sin q =\frac{\cos(p-q)-\cos(p+q)}{2}$.

3. use the double angle formula for sine.