1/42+1/56+1/72+1/90(有简算的要简算)

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1/42+1/56+1/72+1/90(有简算的要简算)
1/42+1/56+1/72+1/90(有简算的要简算)

1/42+1/56+1/72+1/90(有简算的要简算)
1/42+1/56+1/72+1/90
=(1/6-1/7)+(1/7-1/8)+(1/8-1/9)+(1/9-1/10)
=1/6-1/7+1/7-1/8+1/8-1/9+1/9-1/10
=1/6-1/10
=1/15