Đáp án:
$\begin{array}{l}
a)\dfrac{{a - 2\sqrt {ab} + b}}{{\sqrt a - \sqrt b }}\\
= \dfrac{{{{\left( {\sqrt a - \sqrt b } \right)}^2}}}{{\sqrt a - \sqrt b }}\\
= \sqrt a - \sqrt b \\
b)\dfrac{{\sqrt {x - 3} }}{{\sqrt {\sqrt x - \sqrt 3 } }}:\dfrac{{\sqrt {\sqrt x - \sqrt 3 } }}{{\sqrt 3 }}\\
= \dfrac{{\sqrt {\left( {\sqrt x - \sqrt 3 } \right)\left( {\sqrt x + \sqrt 3 } \right)} }}{{\sqrt 3 .\sqrt {\sqrt x - \sqrt 3 } .\sqrt {\sqrt x + \sqrt 3 } }}\\
= \dfrac{1}{{\sqrt 3 }}\\
= \dfrac{{\sqrt 3 }}{3}
\end{array}$
