# Finding the differential of the function

 P: 1,236 $$y = \cos(\pi) x$$ $$\frac{dy}{dx} = \cos (\pi)$$ $$y = \cos(\pi x)$$ $$\frac{dy}{dx} = -\pi \sin \pi x$$. You are using the chain rule. $$dy = -\pi \sin \pi x \ dx$$
 P: 57 Actually it's just written as y = cos pi x. Which I think is basically the same as $$y = \cos\pi x$$ But I understand what you are saying anyways. :D