几道巧算题求解啊1/2+1/6+1/12+1/20+1/30+1/42

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几道巧算题求解啊1/2+1/6+1/12+1/20+1/30+1/42
几道巧算题求解啊
1/2+1/6+1/12+1/20+1/30+1/42

几道巧算题求解啊1/2+1/6+1/12+1/20+1/30+1/42
1/2+1/6+1/12+1/20+1/30+1/42
=1-1/2+1/2-1/3+1/3-1/4+.+1/6-1/7
=1-1/7
=6/7;
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1/2+1/6+1/12+1/20+1/30+1/42
=(1-1/2)+(1/2-1/3)+(1/3-1/4)+(1/4-1/5)+(1/5-1/6)+(1/6-1/7)
=1-1/2+1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7
=1-(1/2-1/2)-(1/3-1/3)-(1/4-1/4)-(1/5-1/5)-(1/6-1...

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1/2+1/6+1/12+1/20+1/30+1/42
=(1-1/2)+(1/2-1/3)+(1/3-1/4)+(1/4-1/5)+(1/5-1/6)+(1/6-1/7)
=1-1/2+1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7
=1-(1/2-1/2)-(1/3-1/3)-(1/4-1/4)-(1/5-1/5)-(1/6-1/6)-1/7
=1-0-0-0-0-0-1/7
=6/7

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