=（1-1/9)-(1-7/8)+(1-1/7)-(1-1/6)+(1-1/5)-(1-1/4)+(1-1/3)-(1-1/2)
=(1/8-1/9)+(1/6-1/7)+(1/4-1/5)+(1/2-1/3)
=1/72+1/42+1/20+1/6
=1/72+1/20+8/42=1/72+1/20+4/21
=1/72+(21+80)/420=1/72+101/420
=35/2520+606/2520=641/2520
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