The quantity y is partly constant varies as the square x.write down the relationship between X and y.when X=1,y=11 and when X=2,y=5.find the value of y when X=4

Answer 1

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The quantity y is partly constant AND varies as the square x>

I will assume you did.
so y = a x^2 + c
when x=1, y=11 ---> 11 = a + c
when x=2, y=5 ---> 5 = 4a + c
subtract those two equations:
-6 = 3a
a = -2, then c = 13

y = -2x^2 + 13

plug in x = 4