14. 5 (1/a √ab)⁸.

$$( \frac{1}{a} \sqrt[4]{ab} )^8 = ( \frac{1}{a} (ab)^{\frac{1}{4}} )^8 = ( \frac{a^{\frac{1}{4}}b^{\frac{1}{4}}}{a} )^8 = ( a^{\frac{1}{4}-1}b^{\frac{1}{4}} )^8 = ( a^{-\frac{3}{4}}b^{\frac{1}{4}} )^8 = a^{-\frac{3}{4} \cdot 8}b^{\frac{1}{4} \cdot 8} = a^{-6}b^2 = \frac{b^2}{a^6}$$

Ответ: $$\frac{b^2}{a^6}$$