Đáp án:
$\begin{array}{l}
a)Dkxd:0 \le x \ne \dfrac{1}{9}\\
P = \left( {1 - \dfrac{{2\sqrt x }}{{3\sqrt x + 1}} - \dfrac{{\sqrt x + 1}}{{9x - 1}}} \right):\left( {\dfrac{{9\sqrt x + 6}}{{3\sqrt x + 1}} - 3} \right)\\
= \dfrac{{9x - 1 - 2\sqrt x \left( {3\sqrt x - 1} \right) - \sqrt x - 1}}{{\left( {3\sqrt x + 1} \right)\left( {3\sqrt x - 1} \right)}}\\
.\dfrac{{3\sqrt x + 1}}{{9\sqrt x + 6 - 9\sqrt x - 3}}\\
= \dfrac{{9x - 1 - 6x + 2\sqrt x - \sqrt x - 1}}{{3\sqrt x - 1}}.\dfrac{1}{3}\\
= \dfrac{{3x + \sqrt x - 2}}{{3.\left( {3\sqrt x - 1} \right)}}\\
b)P = \dfrac{6}{5}\\
\Rightarrow \dfrac{{3x + \sqrt x - 2}}{{3.\left( {3\sqrt x - 1} \right)}} = \dfrac{6}{5}\\
\Rightarrow 18\left( {3\sqrt x - 1} \right) = 15x + 5\sqrt x - 10\\
\Rightarrow 15x - 49\sqrt x + 8 = 0\\
\Rightarrow \left[ \begin{array}{l}
\sqrt x = 3,09\\
\sqrt x = 0,17
\end{array} \right.\\
\Rightarrow \left[ \begin{array}{l}
x = 9,5\\
x = 0,03
\end{array} \right.
\end{array}$
