Question 20

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Jambmaths question: 

Make r the subject of the formula $\frac{x}{r+a}=\frac{a}{r}$

Option A: 

$\frac{{{a}^{2}}}{x-a}$

Option B: 

$\frac{{{a}^{2}}}{x+a}$

Option C: 

$\frac{a}{x-a}$ 

Option D: 

$\frac{a}{x+a}$

Jamb Maths Solution: 

$\begin{align}  & \frac{x}{r+a}=\frac{a}{r} \\ & rx=a(r+a)\text{    (cross multiply)} \\ & rx=ar+{{a}^{2}} \\ & rx-ar={{a}^{2}}\text{     (collect the like terms)} \\ & r(x-a)={{a}^{2}}\text{    (factorize }r\text{ out)} \\ & r=\frac{{{a}^{2}}}{x-a} \\\end{align}$

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