If f (x) varies directly with x2, and f (x) = 40 when x = 2, find the value of f (5).

For a simple, direct relationship, the general equation is f (x) = kx, where k is some constant and in these problems you have to find the value of k.

f (x) = x^2

If f (2) = 40 then f (5) = 100 because f (1) = 20