`xy+yz+xz=xyz`
`⇔1/x+1/y+1/z=1`
`P=(x^2)/(9z+zx^2)+(y^2)/(9x+y^2x)+(z^2)/(9y+yz^2)`
`⇔P=(x^2+9-9)/(z(9+x^2))+(y^2+9-9)/(x(9+y^2))+(z^2+9-9)/(y(z^2+9))`
`⇔P=1/z-(9)/(z(9+x^2))+1/x -(9)/(x(9+y^2))+1/y -(9)/(y(z^2+9))`
`⇔P=(1/x+1/y+1/z)-9(1/(9z+zx^2)+1/(9x+y^2x)+1/(9y+yz^2))`
`⇔P≥1-9(1/(2\sqrt(9z^2x^2))+1/(2\sqrt(9y^2x^2))+1/(2\sqrt(9z^2y^2)))`
`⇔P≥1-9/2(1/(3xy)+1/(3yz)+1/(3xz))`
`⇔P≥1-3/2(1/(xy)+1/(yz)+1/(xz))`
`⇔P≥1-1/2(1/(y)+1/(z)+1/(x))^2`
`⇔P≥1-1/2 .1`
`⇔P≥1/2`
`''=''`xẩy ra khi :
`x=y=z=3`
