JAMB 2012 · UTME · Q38

Find $\dfrac{dy}{dx}$ if $y = \cos x$ .

Worked solution (try it first)

The derivative of $\cos x$ is $-\sin x$ , option B.
Check: the graph of $\cos x$ slopes downwards just after $x = 0$ , where $\sin x$ is positive, so the derivative must be negative there.