12/1+20/1+30/1+42/1+56/1+72/+90/1 等于几?
问题描述：
12/1+20/1+30/1+42/1+56/1+72/+90/1 等于几?
答
1/12+1/20+1/30+1/42+1/56+1/72+1/90
=（1/3-1/4）+（1/4-1/5）+(1/5-1/6)+(1/6-1/7)+(1/7-1/8)+(1/8-1/9)+(1/9-1/10)
=1/3-1/10
=7/30