GWD21-2

meckychen

Q2:

The sum of the first k positive integers is equal toK(K+1)/2.  What is the sum of the integers from n to m
, inclusive, where 0 < n < m ?
A. m(m+1)/2-(n+1)(n+2)/2B.m(m+1)/2-n(n+1)/2

C.m(m+1)/2-n(n-1)/2

D.(m-1)m/2-(n+1)(n+2)/2

E. (m-1)m/2-n(n+1)/2

Answer is C How to calculate? Thank you.

meckychen

up

a10285

题意是  “1到m的和"- "1到n-1的和"=   n到m的和
  因此答案是m(m+1)/2-n(n-1)/2

meckychen

恩。明白了。谢谢你。