证明:
(1)、由平均值不等式,有
$\because 1\leq \sqrt[n]{n}=(\underset{n-2}{\underbrace{1\cdot 1\cdot \cdots 1}}\cdot \sqrt{n}\cdot \sqrt{n})^\frac{1}{n}\leq \frac{(n-2)+2\sqrt{n}}{n}=1+\frac{2(\sqrt{n}-1)}{n}.$
$\therefore 0\leq \sqrt[n]{n}-1\leq \frac{2}{\sqrt{n}},$
$\forall \varepsilon > 0,N=[\frac{4}{\varepsilon ^2}],n> N,s.t.$
$|\sqrt[n]{n}-1|<\frac{2}{\sqrt{n}}< \frac{2}{\sqrt{N}}< \varepsilon .$
(2)、用单调有界性证明极限存在。令
$S_n=1+\frac{1}{2!}+\frac{1}{3!}+\cdots +\frac{1}{n!},$
$\because S_n< S_{n+1},\therefore S_n\uparrow ,$
又
$\displaystyle \because \frac{1}{k!}< \frac{1}{k(k-1)}=\frac{1}{k-1}-\frac{1}{k},(k\geq 2)$
$\displaystyle \therefore S_n=1+\frac{1}{2!}+\frac{1}{3!}+\cdots +\frac{1}{n!}< 1+\sum_{k=2}^{n}(\frac{1}{k-1}-\frac{1}{k})=3-\frac{1}{n}.$
$\displaystyle \Rightarrow \lim_{n \to \infty }S_n=e< 3.$