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. And you could do u substitution and all the rest if you found something like that useful. But it's in general a good habit, or I guess it's good to be able to do this almost by sight. And you can verify that this derivative is equal to sine of pi x.