Compute 2 + 4 + 6 + 8 + ... + 100.
sum(2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, 32, 34, 36, 38, 40, 42, 44, 46, 48, 50, 52, 54, 56, 58, 60, 62, 64, 66, 68, 70, 72, 74, 76, 78, 80, 82, 84, 86, 88, 90, 92, 94, 96, 98, 100) = 2,550
This is best tackled as follows:
2 + 4 + 6 + 8 + ... + 100 = 2*(1 + 2 + 3 + 4 + ... + 50)
The sum of the first n integers is given by n(n+1)/2 so the above sum becomes 2*50*51/2 = 50*51 = 2550
