\(x^4+9x^2=0\\
\Leftrightarrow x^2.\left(x^2+9\right)=0\)
\(\Leftrightarrow\left\{{}\begin{matrix}x^2=0\Leftrightarrow x=0\\\left(x^2+9\right)0\Leftrightarrow\left(x+3\right)^2=0\Leftrightarrow x+3=0\Leftrightarrow x=-3\end{matrix}\right.\)
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