一减六分之五加十二分之七减二十分之九加三十分之十一减四十二分之十三加五十六分十五

5/6+7/12-9/20+11/30-13/42+15/56+17/72-19/90
(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)
=1/2+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
=1/2+1/3+1/3+1/8+1/8-1/10
=1/2+2/3+1/4-1/10
=7/6+1/4-1/10
=15/12-1/10=5/4-1/10=(25-2)/20=23/20=1。15