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求数列极限数列lim[1/(1×2×3)+1/(2×3×4)+1/(3×4×5)+.+1/(n-1)(n)(n+1)要解

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求数列极限
数列lim[1/(1×2×3)+1/(2×3×4)+1/(3×4×5)+.+1/(n-1)(n)(n+1)要解法
求数列极限数列lim[1/(1×2×3)+1/(2×3×4)+1/(3×4×5)+.+1/(n-1)(n)(n+1)要解
2/(k-1)(k)(k+1)=[1/(k-1)-1/k]-[1/k-1/(k+1)] (k∈N)
把上式看成两部分,令k=2~n,并把各式求和,即为:
2/(1×2×3)+2/(2×3×4)+2/(3×4×5)+.+2/(n-1)(n)(n+1)
=[1/1-1/2+1/2-1/3+……+1/(n-1)-1/n] - [1/2-1/3+1/3-1/4+1/4-1/5+……+1/n-1/(n+1)]
=(1-1/n)-[1/2-1/(n+1)]
=1/2+1/(n+1)-1/n
=1/2-1/[n(n+1)]
原式=(1/2)lim{1/2-1/[n(n+1)]}=1/4 (n→∞)