Well, I put the negative there so that when I take the derivative of the cosine of pi x, I would get a negative sign. But that negative will cancel out the negative to give me a positive here. And you say, why did you put a 1 over pi here? Well, when you take the derivative of this thing using the chain rule, you take the derivative of the pi x, you'll get pi that you would multiply everything by. And then you would get negative sine of pi x. And that pi doesn't show up here, so I need something for it to cancel out with. And that's what this 1 over pi is going to cancel out with.