Đk: \(x\ge-3\)
Pt \(\Leftrightarrow4\left(x^2+18\right)^2=49\left(x^3+27\right)\)
\(\Leftrightarrow4x^4-49x^3+144x^2-27=0\)
\(\Leftrightarrow\left(x^2-7x-3\right)\left(4x^2-21x+9\right)=0\)
\(\Leftrightarrow\left[{}\begin{matrix}x=\dfrac{7+\sqrt{61}}{2}\\x=\dfrac{7-\sqrt{61}}{2}\\x=\dfrac{21+3\sqrt{33}}{8}\\x=\dfrac{21-3\sqrt{33}}{8}\end{matrix}\right.\)
Vậy...
ĐKXĐ: \(x\ge-3\).
\(PT\Leftrightarrow2\left(x^2+18\right)=7\sqrt{\left(x+3\right)\left(x^2-3x+9\right)}\). (*)
Đặt \(\sqrt{x+3}=a;\sqrt{x^2-3x+9}=b\left(a,b\ge0\right)\).
\(\left(\cdot\right)\Leftrightarrow2\left(b^2+3a^2\right)=7ab\Leftrightarrow6a^2-7ab+2b^2=0\)
\(\Leftrightarrow\left(3a-2b\right)\left(2a-b\right)=0\Leftrightarrow\left[{}\begin{matrix}3a=2b\\2a=b\end{matrix}\right.\).
+) \(3a=2b\Leftrightarrow3\sqrt{x+3}=2\sqrt{x^2-3x+9}\Leftrightarrow4\left(x^2-3x+9\right)=9\left(x+3\right)\Leftrightarrow4x^2-12x+36=9x+27\Leftrightarrow4x^2-21x+9=0\Leftrightarrow x=\dfrac{21\pm3\sqrt{33}}{8}\). (TMĐK)
+) \(2a=b\Leftrightarrow4\left(x+3\right)=x^2-3x+9\Leftrightarrow x^2-7x-3=0\Leftrightarrow x=\dfrac{7\pm\sqrt{61}}{2}\left(TMĐK\right)\).
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