Giải thích các bước giải:
$A = \dfrac{4x}{\sqrt{x} - 3}$ (ĐK $x \geq 0$)
Ta có: $A < 0$
$\Leftrightarrow \dfrac{4x}{\sqrt{x} - 3} < 0$
$\Leftrightarrow \left[ \begin{array}{l}\left\{\begin{matrix}4x > 0\\ \sqrt{x} - 3 < 0 \end{matrix}\right.\\\left\{\begin{matrix}4x < 0\\ \sqrt{x} - 3 > 0\end{matrix}\right.\end{array} \right.$
$\Leftrightarrow \left[ \begin{array}{l}\left\{\begin{matrix}x > 0\\ x < 9 \end{matrix}\right.\\\left\{\begin{matrix}x < 0\\ x > 9\end{matrix} (L)\right.\end{array}\right.$
$\Leftrightarrow 0 < x < 9$
