1 + 2 + 3 + ........... + 833 + 834 + 835 = N
835 + 834 + 833 + ........... + 3 + 2 + 1 = N
If we add these two reversed equations, we get:
836 + 836 + 836 + ........... + 836 + 836 + 836 = 2N
and as there are 835 terms, then:
835 x 836 = 2N
698060 = 2N
hence the sum, N, of:
1 + 2 + 3 + ........... + 833 + 834 +835 = 698060/2 = 349030