Ta có bổ đề$n^n\geq(n+1)^{n-1}$ với $n\geq1$ tại đây
$<=>n!^n>(n+1)!^{n-1}<=>1\leq \frac{n!.n}{n\sqrt[n]{(n+1)!}^{n-1}}$
Mà $\sqrt[n]{(n+1)!}>\sqrt[n]{n!}=>n\sqrt[n]{(n+1)!}^{n-1}>\sqrt[n]{(n+1)!}^{n-1}+\sqrt[n]{(n+1)!}^{n-2}.\sqrt[n]{n!}+...+\sqrt[n]{n!}^{n-1}$
$=>\frac{n!.n}{n\sqrt[n]{(n+1)!}^{n-1}}\leq\frac{(n+1)!-n!}{\sqrt[n]{(n+1)!}^{n-1}+\sqrt[n]{(n+1)!}^{n-2}.\sqrt[n]{n!}+...+\sqrt[n]{n!}^{n-1}}$
$=\sqrt[n]{(n+1)!}-\sqrt[n]{n!}=>\sqrt[n]{(n+1)!}-\sqrt[n]{n!}\geq1$