Question

Find the following sums ( for letter C)

Answer 1

Each number in the sum is even, so we can remove a factor of 2.

2 + 4 + 6 + 8 + ... + 78 + 80 = 2 (1 + 2 + 3 + 4 + ... + 39 + 40)

Use whatever technique you used in (a) and (b) to compute the sum

1 + 2 + 3 + 4 + ... + 39 + 40

With Gauss's method, for instance, we have

S = 1 + 2 + 3 + ... + 38 + 39 + 40

S = 40 + 39 + 38 + ... + 3 + 2 + 1

2S = (1 + 40) + (2 + 39) + ... + (39 + 2) + (40 + 1) = 40×41

S = 20×21 = 420

Then the sum you want is 2×420 = 840.