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;
```

9

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