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

The answer is m(m+1)/2-n(n-1)/2

高斯求和。大三角-小三角=梯形。减法。
重复计数问题。

谢谢,高兴,懂了