\(\frac{x}{x+yz}+\frac{y}{y+zx}+\frac{z}{z+xy}=\frac{x}{x\left(x+y+z\right)+yz}+\frac{y}{y\left(x+y+z\right)+zx}+\frac{z}{z\left(x+y+z\right)+xy}\)
\(=\text{Σ}\frac{x}{\left(x+y\right)\left(x+z\right)}=\frac{2\left(xy+yz+xz\right)}{\left(x+y\right)\left(y+z\right)\left(x+z\right)}\)(1)
+) CM bổ đề (cái này khá hữu dụng): \(\left(x+y+z\right)\left(xy+yz+xz\right)\ge3\sqrt[3]{xyz}\cdot3\sqrt[3]{x^2y^2z^2}=9xyz\Leftrightarrow\frac{1}{9}\left(x+y+z\right)\left(xy+yz+xz\right)\ge xyz\)
Có \(\left(x+y\right)\left(y+z\right)\left(x+z\right)=\left(x+y+z\right)\left(xy+yz+xz\right)-xyz\ge\frac{8}{9}\left(x+y+z\right)\left(xy+yz+xz\right)\)
Thay vào (1)-> DPCM
Dấu = xảy ra khi x=y=z=1/3
