题目

1-1/2+1/6+1/12+1/20+·········+1/72+1/90巧算

提问时间：2020-12-24

答案

1-1/2+1/2*3+1/3*4+1/4*5……+1/8*9+1/9*10
=1-1/2+1/2-1/3+1/4-1/4+1/4-1/6+……+1/8-1/9+1/9-1/10
=1-1/10
=9/10