Đáp án:
$\begin{array}{l}
x = \frac{{2 - \sqrt 3 }}{2} = \frac{{4 - 2\sqrt 3 }}{4} = \frac{{{{\left( {\sqrt 3 - 1} \right)}^2}}}{{{2^2}}} = {\left( {\frac{{\sqrt 3 - 1}}{2}} \right)^2}\\
\Rightarrow \sqrt x = \frac{{\sqrt 3 - 1}}{2}\\
\Rightarrow A = \frac{{\sqrt x }}{{\sqrt x + 3}} = \frac{{\frac{{\sqrt 3 - 1}}{2}}}{{\frac{{\sqrt 3 - 1}}{2} + 3}} = \frac{{\sqrt 3 - 1}}{{\sqrt 3 - 1 + 6}}\\
= \frac{{\sqrt 3 - 1}}{{\sqrt 3 + 5}} = \frac{{\left( {\sqrt 3 - 1} \right)\left( {\sqrt 3 - 5} \right)}}{{3 - 25}}\\
= \frac{{3 - 6\sqrt 3 + 5}}{{ - 22}} = \frac{{6\sqrt 3 - 8}}{{22}} = \frac{{3\sqrt 3 - 4}}{{11}}
\end{array}$
