二分之一+六分之五+十二分之十一……九十分之八十九=

1/2+5/6+11/12+19/20+……+89/90 =(1-1/2)+(1-1/6)+(1-1/12)+(1-1/20)+.+(1-1/90) =1-1/2+1-1/6+1-1/12+1-1/20+.+1-1/90 =1-(1-1/2)+1-(1/2-1/3)+1-(1/3-1/4)+1-(1/4-1/5)+.+1-(1/9-1/10) =1*8-1+1/2-1/2+1/3-1/3+1/...