求下列方程的精确解：

Answer all questions. You should use a calculator where appropriate.

You must show all necessary working clearly; no marks will be given for unsupported answers from a calculator.

Give non-exact numerical answers correct to 3 significant figures, or 1 decimal place for angles in degrees, unless a different level of accuracy is specified in the question.

请回答所有题目。适当时应使用计算器。必须清楚展示所有必要的计算过程；仅给出计算器答案而没有过程，不予评分。除非题目另有精度要求，非精确数值答案保留 3 位有效数字，以度为单位的角度保留 1 位小数。

先用标准角求反余弦，再孤立反正切。取正切时保留负号。

The principal value of $\cos^{-1}(\sqrt3/2)$ is $\pi/6$. Hence

This angle is in the range of $\tan^{-1}$, so $4x=\tan(-\pi/3)=-\sqrt3$. The exact solution is $x=-\sqrt3/4$.