1又1/3-7/12+9/20-11/30+13/42=1+1/3-(1/3+1/4)+(1/4+1/5)-(1/5+1/6)+(1/6+1/7)=1+1/3-1/3-1/4+1/4+1/5-1/5-1/6+1/6+1/7=1+1/7=1又1/7
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