1/18+1/54+1/108+1/180+1/270+1/378+1/504+1/648+1/810+1/990
=1/9(1/2+1/6+1/12+1/20+1/30+1/42+1/56+1/72+1/90+1/110)
（1/2=1-1/2,1/6=1/2-1/3,1/12=1/3-1/4……）
=1/9(1-1/2+1/2-1/3+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+1/10-1/11)
=1/9(1-1/11)
=10/99