4. Освободите дробь от знака корня в знаменателе: a) $$\frac{7}{2\sqrt{21}}$$ b) $$\frac{22}{\sqrt{13}-\sqrt{2}}$$

a) $$\frac{7}{2\sqrt{21}} = \frac{7\cdot\sqrt{21}}{2\sqrt{21}\cdot\sqrt{21}} = \frac{7\sqrt{21}}{2\cdot21} = \frac{\sqrt{21}}{6}$$ Ответ: $$\frac{\sqrt{21}}{6}$$ b) $$\frac{22}{\sqrt{13}-\sqrt{2}} = \frac{22(\sqrt{13}+\sqrt{2})}{(\sqrt{13}-\sqrt{2})(\sqrt{13}+\sqrt{2})} = \frac{22(\sqrt{13}+\sqrt{2})}{13-2} = \frac{22(\sqrt{13}+\sqrt{2})}{11} = 2(\sqrt{13}+\sqrt{2})$$ Ответ: $$2(\sqrt{13}+\sqrt{2})$$