Ta có: [tex]P=\frac{4x}{\sqrt{x}-3}=\frac{4(x-9)}{\sqrt{x}-3}+\frac{36}{\sqrt{x}-3}=4(\sqrt{x}+3)+\frac{36}{\sqrt{x}-3}=4(\sqrt{x}-3)+\frac{36}{\sqrt{x}-3}+24\geq 2\sqrt{4(\sqrt{x}-3).\frac{36}{\sqrt{x}-3}}+24=48\Rightarrow \sqrt{P}\geq 4\sqrt{3}[/tex]
Dấu "=" xảy ra khi x = 36.