Question 39

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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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