`x^2+3x+1=(x+3)\sqrt{x^2+1}`
`<=>(x+3)\sqrt{x^2+1}-(x+3)x=1`
`<=>(x+3)(\sqrt{x^2+1}-x)=1`
`<=>(x+3)\frac{x^2+1-x^2}{\sqrt{x^2+1}+x}=1`
`<=>x+3=\sqrt{x^2+1}+x`
`<=>x^2+1=9`
`<=>x^2-8=0`
`<=>(x-\sqrt{8})(x+\sqrt{8})=0`
`<=>`\(\left[ \begin{array}{l}x-\sqrt{8}=0\\x+\sqrt{8}=0\end{array} \right.\)
`<=>`\(\left[ \begin{array}{l}x=\sqrt{8}\\x=-\sqrt{8}\end{array} \right.\)
Vậy `S={\sqrt{8};-\sqrt{8}}`
