\(x^2+4x>0\)
\(\Rightarrow x\left(x+4\right)>0\)
\(\Rightarrow\left[{}\begin{matrix}\left\{{}\begin{matrix}x>0\\x+4>0\end{matrix}\right.\\\left\{{}\begin{matrix}x< 0\\x+4< 0\end{matrix}\right.\end{matrix}\right.\Rightarrow\left[{}\begin{matrix}\left\{{}\begin{matrix}x>0\\x>-4\end{matrix}\right.\\\left\{{}\begin{matrix}x< 0\\x< -4\end{matrix}\right.\end{matrix}\right.\Rightarrow\left[{}\begin{matrix}x>0\\x< -4\end{matrix}\right.\)
Vậy...................
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\(x^2+4x>0\)
\(\Rightarrow x\left(x+4\right)>0\)
\(\Rightarrow\left[{}\begin{matrix}\left\{{}\begin{matrix}x>0\\x+4>0\Rightarrow x>4\end{matrix}\right.\\\left\{{}\begin{matrix}x< 0\\x< -4\end{matrix}\right.\end{matrix}\right.\)
Vậy \(x>0\) hoặc \(x< 0\)
\(x^2\)+4x>0
=>\(\left(x+2\right)^2-4>0\)
=>\(\left(x+2\right)^2>4\)
<=> x+2 > 2 hoặc x+2 <-2
=> x>0 hoặc x< -4
