28 tháng 9 2021 lúc 22:18
\(\Leftrightarrow\left[{}\begin{matrix}\dfrac{39}{2}-3x^2=\dfrac{15}{2}\\\dfrac{39}{2}-3x^2=-\dfrac{15}{2}\end{matrix}\right.\)
\(\Leftrightarrow\left[{}\begin{matrix}3x^2=12\\3x^2=27\end{matrix}\right.\)\(\Leftrightarrow\left[{}\begin{matrix}x^2=4\\x^2=9\end{matrix}\right.\)\(\Leftrightarrow\left[{}\begin{matrix}x=2\\x=-2\\x=3\\x=-3\end{matrix}\right.\)
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