123456 ...... n items before this arithmetic sequence and a number of (1 + n) * n / 2
123456 ...... before this arithmetic columns and m is the number of items (1 + m) * m / 2
Assuming m> n required size between n and m
Formula (. 1 + m) * m / 2- (n--. 1) n-* / 2 = (n-m +) (m-n-+. 1) / 2
Expand you can directly launch it, you can try to prove it
