\(4\sqrt{9x^2-42x+49}=12\\ \Leftrightarrow\sqrt{\left(3x-7\right)^2}=3\\ \Leftrightarrow\left|3x-7\right|=3\\ \Leftrightarrow\left[{}\begin{matrix}3x-7=3\\3x-7=-3\end{matrix}\right.\\ \Leftrightarrow\left[{}\begin{matrix}3x=10\\3x=4\end{matrix}\right.\\ \Leftrightarrow\left[{}\begin{matrix}x=\dfrac{10}{3}\\x=\dfrac{4}{3}\end{matrix}\right.\)
Vậy \(S=\left\{\dfrac{10}{3};\dfrac{4}{3}\right\}.\)
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