Question 15

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Jambmaths question: 

If x – 3 is directly proportional to the square of y and x = 5 when y = 2, find x when y = 6

Option A: 

30

Option B: 

21

Option C: 

16

Option D: 

12

Jamb Maths Solution: 

$\begin{align}  & (x-3)\propto {{y}^{2}} \\ & x-3=k{{y}^{2}} \\ & \text{when }y=2,x=5 \\ & 5-3=4k \\ & k=\frac{1}{2} \\ & \text{when }y=6 \\ & x-3=\tfrac{1}{2}{{(6)}^{2}} \\ & x-3=18 \\ & x=21 \\\end{align}$

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