$\dfrac{(\sqrt a - \sqrt b)^2 + 4\sqrt{ab}}{\sqrt a + \sqrt b} - \dfrac{a - \sqrt{ab}}{\sqrt a - \sqrt b}$
$= \dfrac{(\sqrt a)^2 - 2\sqrt{a b} + (\sqrt b)^2 + 4\sqrt{ab}}{\sqrt a + \sqrt b} - \dfrac{\sqrt a(\sqrt a- \sqrt{b}}{\sqrt a - \sqrt b}$
$= \dfrac{(\sqrt a + \sqrt b)^2}{\sqrt a + \sqrt b} - \sqrt a$
$= \sqrt a + \sqrt b - \sqrt a = \sqrt b \quad (đpcm)$
