Question

How do you differentiate `cos 2x`?

Answer 1

Use the chain rule.

We let `y = cosu` and `u = 2x`. Then `dy/(du) = -sinu` and `(du)/dx= 2`.

`dy/dx = dy/(du) xx (du)/dx`

`dy/dx = -sinu xx 2`

`dy/dx = -2sinu`

Since `u = 2x`:

`dy/dx = -2sin2x`

Hopefully this helps!