285. $f(x)=\sqrt[4]{x^{3}}$.

285. $f(x)=\sqrt[4]{x^{3}}$. 

The translation is the same as the original text because it is a mathematical expression which is universal and does not change when translated into English. However, if you need a description or explanation of the function in English, please let me know.
Solution. Since $f(x)=\sqrt[4]{x^{3}}=x^{3 / 4}$, then

$$
f^{\prime}(x)=\left(x^{3 / 4}\right)^{\prime}=\frac{3}{4} x^{3 / 4-1}=\frac{3}{4} x^{-1 / 4}=\frac{3}{4 \sqrt[4]{x}}
$$