\(\left(x+1\right)\left(x-4\right)< 0\)
=> \(\left[{}\begin{matrix}\left\{{}\begin{matrix}x+1< 0\\x-4>0\end{matrix}\right.\\\left\{{}\begin{matrix}x+1>0\\x-4< 0\end{matrix}\right.\end{matrix}\right.\) => \(\left[{}\begin{matrix}\left\{{}\begin{matrix}x< -1\\x>4\end{matrix}\right.\\\left\{{}\begin{matrix}x>-1\\x< 4\end{matrix}\right.\end{matrix}\right.\)
Ta loại bỏ trường hợp x < -1 và x > 4
=> \(x>-1;x< 4\)
=> \(4>x>-1\)