1/2+1/6+1/12+1/20+1/30+1/42+1/56+1/72+1/90=多少

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1/2+1/6+1/12+1/20+1/30+1/42+1/56+1/72+1/90=多少
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1/2+1/6+1/12+1/20+1/30+1/42+1/56+1/72+1/90=多少
1/2+1/6+1/12+1/20+1/30+1/42+1/56+1/72+1/90=多少

1/2+1/6+1/12+1/20+1/30+1/42+1/56+1/72+1/90=多少
1/2+1/6+1/12+1/20+1/30+1/42+1/56+1/72+1/90
=1-1/2+1/2-1/3+1/3-1/4.-1/9-1/10
=1-1/10
=9/10

1/2+1/6+1/12+1/20+1/30+1/42+1/56+1/72+1/90
=1/1*2+1/2*3+1/3*4....+1/9*10
=1-1/2+1/2-1/3+1/3...+1/9-1/10
=1-1/10
=9/10 应用公式 1/n（n+1)=1/n-1/（n+1）