What is this equal to? Now, if we were just trying to find the antiderivative of cosine of x, it's pretty straightforward. We know that the derivative with respect to x of sine of x is equal to cosine of x. Cosine of x. But that's not what we have here. We have cosine of pi x. So there is a technique here, you could call it u substitution. You could say u is equal to pi x.