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The quantity Y is partly constant and partly varies as the square of X (a) write down the relationship between X and Y. (b) when X is 1,Y is11 and when X is 2 and Y is 5 find the value of Y when X is 4.

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  1. y = kx^2 + c

    when x=1, y=11 ----> 11 = k + c
    when x=2, y=5 ----> 5 = 4k + c
    subtract them:
    3k = -6
    k = -2
    in k+c=11
    -2+c=11
    c = 13

    so y = -2x^2 + 13
    now simply find y when x = 4

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