Can you show out how have found the sum so quickly? what is the answer?

Question

1+2+3+4+...+98+99+100=

Answer 1

1+2+3+4+...+98+99+100 

= 1     +  2  +  3 + .....................+ 50   [ first 50 numbers]

 +100 + 99 + 98 + .....................+ 49   [ next 50 numbers

                                                            in revers order]

__________________________________

=101 +101 +101+ .....................+ 101     [ 50 terms]

= 50 times of 101

=(100)/2 * 101         [ if n= 100, sum = n(n+1)/2]

= 5050