Simple trick
Let X=1+2+3+...+100
Now add same series in reverse order as
100+99+98+........+1 = X
Now 2X = 101+101+101+101+......100times
So 2X= 101*100
2X= 10100
X=5050
This formula comes like this = n(n+1)/2 where n is the last number of the series