Determine convergence or divergence of ∞Σ(n=1) 2/[n(n+2)] by expressing Sn as a telescoping sum. If convergent, find the sum.?

2016-03-28 5:22 am

回答 (1)

2016-03-28 7:44 am
✔ 最佳答案
Sol
2/n(n+2)=[(n+2)-n]/[n(n+2)]=1/n-1/(n+2)
A=Σ(n=1 to ∞)_2/[n(n+2)]
=lim(x->∞)_[Σ(n=1 to x)_2/[n(n+2)]
=lim(x->∞)_[Σ(n=1 to x)_1/n-1/(n+2)]
=lim(x->∞)_[Σ(n=1 to x)_1/n-Σ(n=1 to x)_1/(n+2)]
Σ(n=1 to x)_1/n-Σ(n=1 to x)_1/(n+2)
=(1/1+1/2+1/3+1`/4+…+1/x)-[1/2+1/3+1/4+…+1/x+1/(x+1)]
=1/1-1/(x+1)
=x/(x+1)
So
A==lim(x->∞)_[x/(x+1)]=1


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