1+2+3+4+...+98+99+100
= 1 + 2 + 3 + .....................+ 50 [ first 50 numbers]
+100 + 99 + 98 + .....................+ 49 [ next 50 numbers
in revers order]
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=101 +101 +101+ .....................+ 101 [ 50 terms]
= 50 times of 101
=(100)/2 * 101 [ if n= 100, sum = n(n+1)/2]
= 5050