$P=\frac{9x}{yz}+\frac{9x}{yz}+\frac{9x}{yz}+\frac{9x}{yz}+\frac{y}{xz}+\frac{y}{xz}+\frac z{xy}$$\ge 7\sqrt[7]{\frac{9^4.x^4.y^2.z}{x^3.y^5.z^6}}=7\sqrt[7]{\frac{9^4.x}{y^3z^5}} \ge 7\sqrt[7]{\frac{9^4.1}{3^3.3^5}}=7$
Dấu "=" xảy ra $\Leftrightarrow x=1,y=z=3$