Mọi người giúp mik ạ

Đáp án:

$\begin{array}{l}
Dkxd:x > 0;x \ne 4\\
a)P = \left( {\dfrac{{\sqrt x }}{{\sqrt x  - 2}} + \dfrac{{\sqrt x }}{{\sqrt x  + 2}}} \right).\dfrac{{x - 4}}{{\sqrt {4x} }}\\
 = \dfrac{{\sqrt x \left( {\sqrt x  + 2} \right) + \sqrt x \left( {\sqrt x  - 2} \right)}}{{\left( {\sqrt x  - 2} \right)\left( {\sqrt x  + 2} \right)}}.\dfrac{{\left( {\sqrt x  - 2} \right)\left( {\sqrt x  + 2} \right)}}{{2\sqrt x }}\\
 = \dfrac{{x + 2\sqrt x  + x - 2\sqrt x }}{1}.\dfrac{1}{{2\sqrt x }}\\
 = \dfrac{{2x}}{{2\sqrt x }}\\
 = \sqrt x \\
b)P = 3\\
 \Leftrightarrow \sqrt x  = 3\\
 \Leftrightarrow x = 9\left( {tmdk} \right)\\
Vay\,x = 9
\end{array}$