2+99= 10 1

3+98= 10 1

.

.

.

50+5 1= 10 1

So the number 100 can be divided into 50 groups, and each group is 10 1.

1+2+3+4+5........+99+ 100= 10 1*50=5050

Actually, this is a arithmetic progression, which is (1+1kloc-0/) *100/2 = 5050 according to the formula.