1-1/2+1/6+1/12+1/20+·········+1/72+1/90巧算
问题描述：
1-1/2+1/6+1/12+1/20+·········+1/72+1/90巧算
答
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