`P=\sqrt{x}/(x+\sqrt{x}+1)`
`P<1/3`
`->\sqrt{x}/(x+\sqrt{x}+1)<1/3`
`->\sqrt{x}/(x+\sqrt{x}+1)-1/3<0`
`->(3\sqrt{x}-x-\sqrt{x}-1)/(x+\sqrt{x}+1)<0`
`->-(x-2\sqrt{x}+1)/(x+\sqrt{x}+1)<0`
`->(\sqrt{x}-1)^2/(x+\sqrt{x}+1)<0`
`x>=0`
`->x+\sqrt{x}+1>0`
`(\sqrt{x}-1)^2>0`
`->-(\sqrt{x}-1)^2<0`
`-> P<1/3 ∀xTMĐK`