A function is defined by f(0) = 0 and f(n) = f(n − 1) + 2n, for n > 0. Find f(4).
f(1) = f(1- 1) + 2(1) = f(0) + 2 = 2
f(2) = f(2 - 1) + 2(2) = f(1) + 4 = 2 + 4 = 6
f(3) = f(3 - 1) + 2(3) = f(2) + 6 = 6 + 6 = 12
f(4) = f(4 - 1) + 2(4) + f(3) + 8 = 12 + 8 = 20
