1/12+1/20+1/30+1/42+1/56+1/72+1/90

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1/12+1/20+1/30+1/42+1/56+1/72+1/901/12+1/20+1/30+1/42+1/56+1/72+1/901/12+1/20+1/30+1/42+1/56+1/72+1/

1/12+1/20+1/30+1/42+1/56+1/72+1/90
1/12+1/20+1/30+1/42+1/56+1/72+1/90

1/12+1/20+1/30+1/42+1/56+1/72+1/90
=1/3*4+1/4*5+1/5*6+1/6*7+1/7*8+1/8*9+1/9*10
=1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7+1/7-1/8+1/8-1/9+1/9-1/10
=1/3-1/10
=7/30
【通项:
1/n(n+1)=1/n-1/n+1】

1/12=1/3-(1/4)
1/20=1/4-(1/5)
……
1/90=1/9-(1/10)
1/12+1/20+1/30+1/42+1/56+1/72+1/90
=1/3-(1/4)+(1/4)-(1/5)+……+(1/9)-(1/10)
=1/3-(1/10)
=7/30

1/12+1/20+1/30+1/42+1/56+1/72+1/90
=(1/3-1/4)+(1/4-1/5)+……+(1/9-1/10)
=1/3-1/10
=7/30

因为1/12 =1/(3×4)=1/3-1/4
所以
1/12+1/20+1/30+1/42+1/56+1/72+1/90
=1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7+1/7-1/8+1/8-1/9+1/9-1/10
=1/3-1/10
=7/30