1又1/2-2又5/6+3又1/12-4又19/20+5又1/30-6又41/42+7又1/56-8又71/72+9又1/90 =(1+1/2)+(1/6-3)+(3+1/12)+(1/20-5)+(5+1/30)+(1/42-7)+(7+1/56)+(1/72-9)+(9+1/90) =(1-3+3-5+5-7+7-9+9)+(1/2+1/6+1/12+1/20+1/30+1/42+1/56+1/72+1/90) =1+(1/2+1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/+1/6-1/7+1/7-1/8+1/8-1/9+1/9-1/10) =2-1/10 =19/10
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