Math CentralQuandaries & Queries


Question from Kimberly, a student:

If y is directly proportional to the square of x,
and x=2 when y=12, then what is the value of y when x=3?


Hi Kimberly,

The statement that y is directly proportional to the square of x means that the proportion of y to x2 is always the same number. That is there is a constant k so that

y/x2 = k

You are told that when x = 2, y = 12. Substitute these values into the above equation and solve for k. You know that k is is a constant so when x = 3, k still has the same value so you can substitute x = 3 into y/x2 = k and solve for y.

I hope this helps,

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