Question 38

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Jambmaths question: 

If $y=3\sin (-4x),\frac{dy}{dx}\text{ is }$

Option A: 

$12x\cos (4x)$

Option B: 

$12x\cos (4x)$

Option C: 

$-12x\cos (-4x)$

Option D: 

$12\sin (-4x)$

Jamb Maths Solution: 

$\begin{align}  & y=3\sin (-4x) \\ & \text{Using chain rule method, let }u=-4x \\ & y=3\sin u \\ & \frac{dy}{du}=3\cos u,\text{   }\frac{du}{dx}=-4 \\ & \frac{dy}{dx}=\frac{dy}{du}\times \frac{du}{dx}=3\cos u\times (-4)=-12\cos u \\ & \frac{dy}{dx}=-12\cos (-4x) \\\end{align}$

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