Question

i really need help with this. derivative of y=cos(2x), then y=?

Answer 1

d/dx cos (2x) d/dx(2x)
-sin(2x) *2

Answer 2

Use the chain rule:
\[y'=\frac{d}{d(2x)}(\cos(2x)) \times \frac{d}{dx}(2x)=-\sin(2x) \times 2=-2\sin(2x)\]