Question 13

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Jambmaths question: 

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

Option A: 

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

Option B: 

$12x\cos (4x)$

Option C: 

$12\sin (-4x)$

Option D: 

$-12\cos (-4x)$

Jamb Maths Solution: 

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

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