Trần Đức Huy
15/05/2018 at 08:17-
Alone 15/05/2018 at 13:53\(x^4-8x^2-9=0\Leftrightarrow x^4+x^2-9x^2-9=0\)
\(\Leftrightarrow x^2\left(x^2+1\right)-9\left(x^2+1\right)=0\)
\(\Leftrightarrow\left(x^2-9\right)\left(x^2+1\right)=0\Leftrightarrow\left(x-3\right)\left(x+3\right)\left(x^2+1\right)=0\)
Because \(x^2+1\ne0\) so \(\left[{}\begin{matrix}x-3=0\\x+3=0\end{matrix}\right.\)\(\Leftrightarrow\left[{}\begin{matrix}x=3\\x=-3\end{matrix}\right.\)
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