Problem

甲、填充題：共 8 題，每題 8 分，共 64 分。請在答案卷上列出題號依序作答。

5. Find the directional derivative of $f(x,y)=\tan^{-1}(x/y)$ at $P_0(1,1)$ in the direction of $\mathbf{v}=\dfrac{\sqrt2}{2}\mathbf{i}+\dfrac{\sqrt2}{2}\mathbf{j}$. Answer : \underline{\quad(1)\quad}