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1/1x2x3x4+1/2x3x4x5+1/3x4x5x6+1/4x5x6x7+1/5x6x7...

1/(1×2×3×4)+1/(2×3×4×5)+1/(3×4×5×6)+...+1/(7×8×9×10) =(1/3)×[1/(1×2×3)-1/(2×3×4)]+(1/3)×[1/(2×3×4)-1/(3×4×5)]+(1/3)×[1/(3×4×5)-1/(4×5×6)]+...+(1/3)×[1/(7×8×9)-1/(8×9×10)] =(1/3)×[1/(1×2×3)-1/(2×3×4)+1/(2×3×4)-1/(3×4×5)...

6/7

你在算算 是210个6

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