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1x2×3十2x3x4十3x4X5+…十9X10x11的值是的少

1/n(n+1)(n+2)=1/2*[1/n-2/(n+1)+1/(n+2)] 原式=1/2*(1-2*1/2+1/3+1/2-2*1/3+1/4+....+1/9-2*1/10+1/11) =1/2*(1-1/2-1/10+1/11)=27/110

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