ĐKXĐ: $a>0$ và $a\neq\{1;4\}$
a) $Q=\Bigg(\dfrac{1}{\sqrt[]{a}-1}-\dfrac{1}{\sqrt[]{a}}\Bigg):\Bigg(\dfrac{\sqrt[]{a}+1}{\sqrt[]{a}-2}-\dfrac{\sqrt[]{a}+2}{\sqrt[]{a}-1}\Bigg)$
$=\Bigg[\dfrac{\sqrt[]{a}-\sqrt[]{a}+1}{\sqrt[]{a}(\sqrt[]{a}-1)}\Bigg]:\dfrac{(\sqrt[]{a}+1)(\sqrt[]{a}-1)-(\sqrt[]{a}+2)(\sqrt[]{a}-2)}{(\sqrt[]{a}-2)(\sqrt[]{a}-1)}$
$=\dfrac{1}{\sqrt[]{a}(\sqrt[]{a}-1)}.\dfrac{(\sqrt[]{a}-2)(\sqrt[]{a}-1)}{3}$
$=\dfrac{\sqrt[]{a}-2}{3\sqrt[]{a}}$
b) $Q>0$
$↔ \dfrac{\sqrt[]{a}-2}{3\sqrt[]{a}}>0$
$↔ a>0$ và $\sqrt[]{a}-2>0$
$↔ a>0$ và $\sqrt[]{a}>2$
$↔ a>4$
