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. If you don't know how to do that, you could still try to think about, think this through, where we could say, all right, well, maybe it involves sine of pi x somehow. So the derivative with respect to x of sine of pi x would be what? Well, we would use the chain rule.