`\sqrt{4x^2 + 1/9 - 4/3x} = 3`
`⇔ 4x^2 + 1/9 - 4/3x = 9`
`⇔ 36x^2 + 1 - 12x = 81`
`⇔ 36x^2 - 12x - 80 = 0`
`⇔ 4(9x^2 - 3x - 20) = 0`
`⇔ 4(3x-5)(3x+4) = 0`
`⇔`\(\left[ \begin{array}{l}3x-5=0\\3x+4=0\end{array} \right.\)
`⇔`\(\left[ \begin{array}{l}x=\dfrac{5}{3}\\x=-\dfrac{4}{3}\end{array} \right.\)
Vậy `S = {5/3,-4/3}`
