câu 7$1+x=1+\frac{x}{3}+\frac{x}{3}+\frac{x}{3}\geq4 \sqrt[4]{\frac{x^{3}}{3^{3}}}$
TT $1+\frac{y}{x}\geq 4 \sqrt[4]{\frac{y^{3}}{3^{3}x^{3}}}$
$1+\frac{9}{\sqrt{y}} \geq 4 \sqrt[4]{\frac{3^{3}}{(\sqrt{y})^{3}}}$
$\Rightarrow (1+\frac{9}{\sqrt{y}})^{2} \geq 16 \sqrt[4]{\frac{3^{6}}{y^{3}}}$
$\Rightarrow P\geq 256\sqrt[4]{\frac{x^{3}}{3^{3}}\frac{y^{3}}{3^{3}x^{3}}\frac{3^{6}}{y^{3}}}$
$\Rightarrow P\geq 256$
dấu "=" $\Leftrightarrow x=3; y=9$