fi: is f subscript i

How do I prove that if fi(n) are linear functionsi.e., that fi(n) = (n) for all fi(n)then Summation from i=1 to n, i*fi(n) = (n^3 ). I'm not required to use the formal definition of Big-Oh.

I'm having trouble understanding how to apply the big oh, theta, and omega concepts to functions instead of just variables, which I guess is why I'm having trouble with this problem.