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/(2×3×4)-1/(3×4×5)+1/(3×4×5)-1/(4×5×6)+……+1/(7×8×9）-1/(8×9×10)】
=1/3×【1/(1×2×3)+······+1/(8×9×10)】
=1/3×【1/6-1/720】
=1/3×119/720
=119/2160
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