Giải thích các bước giải:
Ta có:
\(\begin{array}{l}
a,\\
DKXD:\,\,\,\left\{ \begin{array}{l}
x \ge 0\\
\sqrt x + 2 \ne 0\\
\sqrt x - 2 \ne 0\\
x - 4\sqrt x + 4 \ne 0
\end{array} \right. \Leftrightarrow \left\{ \begin{array}{l}
x \ge 0\\
x \ne 4
\end{array} \right.\\
b,\\
A = \left( {\dfrac{3}{{\sqrt x + 2}} + \dfrac{3}{{\sqrt x - 2}}} \right):\dfrac{{12\sqrt x }}{{x - 4\sqrt x + 4}}\\
= \dfrac{{3.\left( {\sqrt x - 2} \right) + 3.\left( {\sqrt x + 2} \right)}}{{\left( {\sqrt x + 2} \right)\left( {\sqrt x - 2} \right)}}:\dfrac{{12\sqrt x }}{{{{\left( {\sqrt x - 2} \right)}^2}}}\\
= \dfrac{{6\sqrt x }}{{\left( {\sqrt x + 2} \right)\left( {\sqrt x - 2} \right)}}.\dfrac{{{{\left( {\sqrt x - 2} \right)}^2}}}{{12\sqrt x }}\\
= \dfrac{{\sqrt x - 2}}{{2.\left( {\sqrt x + 2} \right)}}
\end{array}\)
