Jambmaths question:
If $y=\cos 3x$ , find $\tfrac{dy}{dx}$
Option A:
$-\tfrac{1}{3}\sin 3x$
Option B:
$3\sin 3x$
Option C:
$-3\sin 3x$
Option D:
$\tfrac{1}{3}\sin 3x$
Jamb Maths Solution:
$\begin{align} & y=\cos 3x \\ & \text{let }u=3x,\text{ }\frac{dx}{du}=3 \\ & y=\cos u,\text{ }\frac{dy}{du}=-\sin u \\ & \frac{dy}{dx}=\frac{dy}{du}\times \frac{du}{dx}=-\sin u\times 3=-3sinu \\ & \frac{dy}{dx}=-3\sin 3x \\\end{align}$
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