[Solved] algorithm for the sum of integer peer from n to n [closed]

For 1 to n the formula is sum =n(n+1)/2 , so from n1 to n2 you have to calculate sum of n1-1 terms snd sum of n2 terms and then subtract.

sum_of_n1_to_n2 = n2(n2+1)/2  - (n1-1)((n1-1) +1)/2;