1加上2分之3加上6分之7加上12分之13加上...30分之31
=1+1+1/2+1+1/6+1+1/12+1+1/20+1+1/30
=1x6+1/2+1/6+1/12+1/20+1/30
=6+1/2+1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6
=7-1/6
=6又5/6
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