Đáp án:
$\begin{array}{l}
a)Dkxd:\left\{ \begin{array}{l}
a \ge 0\\
a \ne 9
\end{array} \right.\\
b)A = \frac{{a - 3\sqrt a }}{{\sqrt a - 3}} - \frac{{a + 4\sqrt a + 3}}{{\sqrt a + 3}}\\
= \frac{{\sqrt a \left( {\sqrt a - 3} \right)}}{{\sqrt a - 3}} - \frac{{\left( {\sqrt a + 1} \right)\left( {\sqrt a + 3} \right)}}{{\sqrt a + 3}}\\
= \sqrt a - \left( {\sqrt a + 1} \right)\\
= \sqrt a - \sqrt a - 1\\
= - 1
\end{array}$