`\sqrt{x^2-3x+5} + x^2 - 3x = 7`
`⇔ x^2 - 3x + 5 = x^4 - 6x^3 - 5x^2 + 42x + 49`
`⇔ x^4 - 6x^3 - 5x^2 + 42x + 49 = x^2 - 3x + 5`
`⇔ x^4 - 6x^3 - 5x^2 + 42x + 44 = x^2 - 3x`
`⇔ x^4 - 6x^3 - 5x^2 + 45x +44 =x^2`
`⇔ x^4 - 6x^3 - 6x^2 + 45x + 44 =0`
`⇔ (x+1)(x-4)(x^2-3x-11)=0`
`⇔`\(\left[ \begin{array}{l}x+1=0\\x-4=0\\x^2-3x-11=0\end{array} \right.\)
`⇔`\(\left[ \begin{array}{l}x=-1\\x=4\\x=\dfrac{3\pm\sqrt{53}}{2}\end{array} \right.\)
Vậy `S ={-1,4,(3\pm\sqrt{53})/2}`
