21. \sqrt[3]{\sqrt[4]{2}}

Ответ:

$$\sqrt[3]{\sqrt[4]{2}} = (2^{\frac{1}{4}})^{\frac{1}{3}} = 2^{\frac{1}{4} \cdot \frac{1}{3}} = 2^{\frac{1}{12}} = \sqrt[12]{2}$$