✔ 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