题目

(1/2)数列1*n,2(n-1),3(n-2),…,n*1的和为（）A.1/6n(n+1)(n+2) B.1/6n(n+1)(2

提问时间：2021-03-11

答案

n * 1　= n(n+1) - n²1*n + 2(n-1) + 3(n-2) + ... + n*1= (1+2+3+...+n)(n+1) - (1² + 2² + 3² + ... + n²)= n(n+1)(n+1)/2 - n(n+1)(2n+1)/6= n(n+1)(n+2)/6