Đặt $f\left( n
\right)={{\left( {{n}^{2}}+n+1 \right)}^{2}}+1.$ Xét dãy$\left( {{u}_{n}}
\right)$:${{u}_{n}}=\frac{f\left(
1 \right).f\left( 3 \right).f\left( 5 \right)...f\left( 2n-1 \right)}{f\left( 2
\right).f\left( 4 \right).f\left( 6 \right)...f\left( 2n \right)}.$ Tính $\lim n\sqrt{{{u}_{n}}}$