Question

Find a formula for the sum of the first n even integers : 2 + 4 + 6 + ... + 2n.

Answer 1

Let S be the sum,

S = 2 + 4 + 6 + ... + 2 (n - 2) + 2 (n - 1) + 2n

Reverse the order of terms:

S = 2n + 2 (n - 1) + 2 (n - 2) + ... + 6 + 4 + 2

Add up terms in the same positions, so that twice the sum is

2S = (2n + 2) + (2n + 2) + (2n + 2) + ... + (2n + 2)

or

2S = n (2n + 2)

Divide both sides by 2 to solve for S :

S = n (n + 1)