`\quad \sqrt{x^2-6x+9}=\sqrt{4-2\sqrt{3}}+\sqrt{4+2\sqrt{3}}`
`<=> \sqrt{x^2-2.x.3+3^2}=\sqrt{3-2\sqrt{3}+1}+\sqrt{3+2\sqrt{3}+1}`
`<=> \sqrt{(x-3)^2}=\sqrt{(\sqrt{3}-1)^2}+\sqrt{(\sqrt{3}+1)^2}`
`<=> |x-3|=|\sqrt{3}-1|+|\sqrt{3}+1|`
`<=> |x-3|=\sqrt{3}-1+\sqrt{3}+1`
`<=> |x-3|=2\sqrt{3}`
`<=>`\(\left[ \begin{array}{l}x-3=2\sqrt{3}\\x-3=-2\sqrt{3}\end{array} \right.\)
`<=>`\(\left[ \begin{array}{l}x=3+2\sqrt{3}\\x=3-2\sqrt{3}\end{array} \right.\)
Vậy `S={3+-2\sqrt{3}}`