5.
lim [(n+2)/n]^(n+1)
=lim (1+2/n)^(2n/2+1)
=lim (1+2/n)^[2*(n/2)]*(1+2/n)
=lim [(1+2/n)^(n/2)]^2 * lim (1+2/n)
=e^2*1
=e^2
6.
lim [(n+2)/(n+1)]^n
=lim (1+1/(n+1))^(n+1-1)
=lim (1+1/(n+1))^(n+1)*(1+1/(n+1))^(-1)
=lim (1+1/(n+1))^(n+1) * lim (1+1/(n+1))^(-1)
=e*1
=e
就是用重要的极限:lim (1+1/n)^n=e,n趋于无穷
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