Hi. As usual, struggling with basic integration stuff I'm already supposed to have mastered. Could somebody point me in the right direction with the following integral? I'm thinking maybe it is a trigonometric type of problem, but that is just a guess. Played around with addition by 0 and multiply by 1 tricks, but nothing seems to have helped so far.

$\displaystyle \int{\sqrt{r^2-x^2}\,dx}$