\(x^2-5x-3\sqrt{3x}+12=0\)
\(\Leftrightarrow\left(x^2-6x+9\right)+\left(x-2\sqrt{3x}+3\right)=0\)
\(\Leftrightarrow\left(x-3\right)^2+\left(\sqrt{x}-\sqrt{3}\right)^2=0\)
\(\Leftrightarrow\hept{\begin{cases}x-3=0\\\sqrt{x}-\sqrt{3}=0\end{cases}}\Leftrightarrow x=3\)
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