Ta có
$C = \left( \dfrac{1}{\sqrt{a} - 1} - \dfrac{1}{\sqrt{a}} \right) : \left( \dfrac{2}{\sqrt{a} - 2} - \dfrac{1}{\sqrt{a}-1} \right)$
$= \left( \dfrac{\sqrt{a} - (\sqrt{a}-1)}{\sqrt{a}(\sqrt{a}-1)} \right) : \left( \dfrac{2(\sqrt{a} - 1) - (\sqrt{a} - 2)}{(\sqrt{a}-2)(\sqrt{a}-1)} \right)$
$= \left( \dfrac{1}{\sqrt{a}(\sqrt{a}-1)} \right) : \left( \dfrac{\sqrt{a}}{ (\sqrt{a}-2)(\sqrt{a}-1)} \right)$
$= \dfrac{1}{\sqrt{a}(\sqrt{a}-1)}. \dfrac{(\sqrt{a}-2)(\sqrt{a}-1)}{\sqrt{a}}$
$= \dfrac{\sqrt{a}-2}{a}$
