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。

4. Find the values of $a$ and $b$ that makes the function

$$f(x)= \begin{cases} \dfrac{2\sin^2 x}{x}, & \text{if }x>0,\\ ax+b\cos x, & \text{if }x\le 0, \end{cases}$$

differentiable at $x=0$. Answer : \underline{\quad(1)\quad}