Освободимся от иррациональности в знаменателе дроби:
$$\frac{1}{\sqrt[4]{8}} = \frac{1}{\sqrt[4]{2^3}} = \frac{1}{\sqrt[4]{2^3}} \cdot \frac{\sqrt[4]{2}}{\sqrt[4]{2}} = \frac{\sqrt[4]{2}}{\sqrt[4]{2^4}} = \frac{\sqrt[4]{2}}{2}$$.
$$\frac{6}{\sqrt{4} + \sqrt{2}} = \frac{6}{2 + \sqrt{2}} = \frac{6}{2 + \sqrt{2}} \cdot \frac{2 - \sqrt{2}}{2 - \sqrt{2}} = \frac{6(2 - \sqrt{2})}{4 - 2} = \frac{6(2 - \sqrt{2})}{2} = 3(2 - \sqrt{2}) = 6 - 3\sqrt{2}$$.
Ответ: 1) $$\frac{\sqrt[4]{2}}{2}$$; 2) $$6 - 3\sqrt{2}$$.