\(\left|x^3+x\right|-\left|9x^2+9\right|=0\)
\(\Leftrightarrow\left|x\left(x^2+1\right)\right|-9\left|x^2+1\right|=0\)
\(\Leftrightarrow\left(\left|x\right|-9\right)\left(x^2+1\right)=0\)
\(\Leftrightarrow\left|x\right|=9\left(x^2+1\ge1>0\right)\Leftrightarrow x=\pm9\)
Vậy ...
\(\left|x^3+x\right|-\left|9x^2+9\right|=0\)
\(TH1:\left\{{}\begin{matrix}\left|x^3+x\right|=0\\\left|9x^2+9\right|=0\end{matrix}\right.\)
\(\text{Vì }9x^2\ge0\)
\(\Rightarrow9x^2+9\ge9\)
\(TH2:\left|x^3+x\right|=\left|9x^2+9\right|\)
\(\Rightarrow\left[{}\begin{matrix}x^3+x=9x^2-9\\x^3+x=9x^2+9\end{matrix}\right.\)
\(\Rightarrow\left[{}\begin{matrix}x^3+x+9x^2+9=0\\x^3+x-9x^2-9=0\end{matrix}\right.\)
\(\Rightarrow\left[{}\begin{matrix}x.\left(x^2+1\right)+9.\left(x^2+1\right)=0\\x.\left(x^2+1\right)-9.\left(x^2+1\right)=0\end{matrix}\right.\)
\(\Leftrightarrow\left[{}\begin{matrix}x=-9\\x=9\end{matrix}\right.\)