Question

What is the derivative of `cos(-x)`?

Answer 1

`d/dxcos(-x) = -sin(x)`

Explanation:

Assuming we know that `d/dxcos(x) = -sin(x)`, we could use the chain rule on `f@g(x)` with `f(x) = cos(x)` and `g(x) = -x`.

However, a slightly nicer method is to note that cosine is an even function, meaning `cos(-x) = cos(x)`. Thus

`d/dxcos(-x) = d/dxcos(x) = -sin(x)`