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If x - 3 is directly proportional to the square... - JAMB Mathematics 2008 Question

If x - 3 is directly proportional to the square of y and x = 5 when y =2, find x when y = 6.
A
30
B
21
C
16
D
12
correct option: b

(x – 3) ∝ y2

X-3 = Ky2

K = X-3 / y2

= 5-2/22

= 2/4

= 1/2

∴X-3 = 1/2y2

X-3 = 1/2(6)2

X-3 = 1/2 x 30/1

X-3 = 18

X = 21

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