1+1/3-7/12+9/20-11/30+13/42-15/56+17/72
=1+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+1/9
=1又1/9
1+1/3-7/12+9/20-11/30+13/42-15/56+17/72
=1+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+1/9=10/9
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