I) Chứng minh rằng:
1. $lim\frac{2^{n}}{2!}=0$
2.$lim\sqrt[n]{\frac{1}{n!}}=0$
3.$lim\frac{n}{a^{n}}=0$
II) Tính lim(Un):
1. $U_{n}=\frac{2}{\sqrt{n^2+1}}+\frac{2}{\sqrt{n^2+2}}+...+\frac{2}{\sqrt{n^2+n}}$
2.$U_{n}=\sqrt{6+\sqrt{6+\sqrt{6+...+\sqrt{6}}}}$
(n dấu căn)