An algebra problem by Phạm Quốc Tâm

I guess this solution is for warming-up only.

$\huge S = \frac{n(a_1 + a_n)}{2} = \frac{50(1+100)}{2} = 5050$

Gauss was given this problem when he was 10 years old as punishment for talking in class. He had already discovered the sum of an A.P. and immediately wrote 5050 on the slate. The teachers response is not known. Ed Gray

Since 1+99= 2+98=...=100. Solution is simple, doesn't require a pen to write: S=50 x 100 + 50 =5050