I need help figuring out this problem.. If f(x) varies directly with x2, and f(x) = 75 when x = 5, find the value of f(4).
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Question

I need help figuring out this problem..  If f(x) varies directly with x2, and f(x) = 75 when x = 5, find the value of f(4). 
(Points : 1)
       100
       50
       48
       25

whpalmer4 · Answer

Direct variation means that $$y = kx$$ if $y$ varies directly with $x$.  Here, we have $f(x)$ varying directly with $x^2$, so our equation would be $f(x) = kx^2$.  Does that make sense?

whpalmer4 · Answer

To solve the problem, plug in $f(x) = 75$ and $x = 5$ in your equation, and solve for the value of $k$.  Then use that value of $k$ in the equation along with $x = 4$ to find the value of $f(4)$.