Question

if f(x)=cos^2(2x), find f'(pi/3)

Answer 1

Using the chain rule, the derivative of f(x) is -2 cos (2x) sin (2x). Evaluating this at pi/3 gives you sqrt(3)/2

Answer 2

I thought you had to multiply an additional 2 using the chain rule?

Answer 3

Oh, I forgot. Nice catch. So the answer would be sqrt(3).

Lesson: never try to do chain rule in your head.