Áp dụng BĐT Bunhia ta có:$(x^2+y+z)(1+y+z)\ge(x+y+z)^2=9 \Rightarrow \dfrac{x}{x^2+y+z}\le\dfrac{x(1+y+z)}{9}$
Tương tự: $\dfrac{y}{x+y^2+z}\le\dfrac{y(1+x+z)}{9},\dfrac{z}{x+y+z^2}\le\dfrac{z(1+x+y)}{9}$
Suy ra:
$VT\le\dfrac{x+y+z+2(xy+yz+zx)}{9}\le\dfrac{x+y+z+\dfrac{2}{3}(x+y+z)^2}{9}=1$
Dấu bằng xảy ra khi: $x=y=z=1$