Chứng minh bất đảng thức sau

$$\begin{gathered} \mathrm{cho} \mathrm{x},\mathrm{y},\mathrm{z} \mathrm{duong}. \mathrm{và} \mathrm{x}+\mathrm{y}+\mathrm{z}=1.\mathrm{CMR}: \\ \frac{\mathrm{x}}{\mathrm{x}+\mathrm{yz}}+\frac{\mathrm{y}}{\mathrm{y}+\mathrm{zx}}+\frac{\mathrm{z}}{\mathrm{z}+\mathrm{xy}}⩽\frac{9}{4} \end{gathered}$$