Question 21

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Jambmaths question: 

The inverse of the function $f(x)=3x+4$ is

Option A: 

$\tfrac{1}{3}(x-4)$

Option B: 

$\tfrac{1}{5}(x-5)$

Option C: 

$\tfrac{1}{4}(x+3)$

Option D: 

$\tfrac{1}{3}(x+4)$

Jamb Maths Solution: 

$\begin{align}  & f(x)=3x+4 \\ & Let\text{ }y=3x+4 \\ & 3x=f(x)-4 \\ & x=\tfrac{1}{3}[f(x)-4] \\ & \text{Now replace back }f(x)\text{ with }x\text{ and }x\text{ with }{{f}^{-1}}(x) \\ & {{f}^{-1}}(x)=\tfrac{1}{3}(x-4)\text{   }\!\!\{\!\!\text{ Where }{{f}^{-1}}(x)\text{ is the inverse }f(x)\} \\\end{align}$

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