JAMB 2014 · UTME · Q39

If $y = \cos 3x$ , find $\dfrac{dy}{dx}$ .

Worked solution (try it first)

Chain rule: $\cos3x$ differentiates to $-\sin3x$ times the derivative of $3x$ , which is 3.
So $\frac{dy}{dx} = -3\sin3x$ , option A.