`~rai~`
`x+3\sqrt{x}+1=0(ĐKXĐ:x≥0)`
`⇔x+2\sqrt{x}+1+\sqrt{x}=0`
`⇔(\sqrt{x}+1)^2+\sqrt{x}=0`
`text{Ta có:}`
`\sqrt{x}≥0`
`⇔\sqrt{x}+1≥1`
`⇔(\sqrt{x}+1)^2≥1`
`⇔(\sqrt{x}+1)^2+\sqrt{x}≥1`
` mà(\sqrt{x}+1)^2+\sqrt{x}=0` `text{(vô lý)}`
$\Rightarrow x\in \varnothing.$
`text{Vậy x∈∅}.`