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