6. $$\left(\frac{1}{16}\right)^{\frac{1}{4}} + \left(\frac{1}{81}\right)^{\frac{1}{4}}$$

$$\left(\frac{1}{16}\right)^{\frac{1}{4}} + \left(\frac{1}{81}\right)^{\frac{1}{4}} = \frac{1^{\frac{1}{4}}}{16^{\frac{1}{4}}} + \frac{1^{\frac{1}{4}}}{81^{\frac{1}{4}}} = \frac{\sqrt[4]{1}}{\sqrt[4]{16}} + \frac{\sqrt[4]{1}}{\sqrt[4]{81}} = \frac{1}{2} + \frac{1}{3} = \frac{3}{6} + \frac{2}{6} = \frac{5}{6}$$

Ответ: $$\frac{5}{6}$$