326. Find the derivative of the function $y=\cos x^{4}$.
Solution. Here $u=x^{4}, y=\cos u$. We find $y_{u}^{\prime}=-\sin u, u_{x}^{\prime}=$ $=4 x^{3}$. Therefore, $y_{x}^{\prime}=y_{u}^{\prime} u_{x}^{\prime}=-4 x^{3} \sin x^{4}$.