In Diagram I below, $ABCDE$ is a regular pentagon, centre $O$ . $OA = OB = 4\text{ cm}$ . (a) State the value of angle $AOB$ . Answer Angle $AOB =$ ___ $^\circ$

Worked solution

1. 计算正五边形的中心角 $\angle AOB$ 的度数

正五边形有 $5$ 条相等边，中心角 $\angle AOB$ 将周角 $360^\circ$ $5$ 等分：
$\angle AOB = \frac{360^\circ}{5} = 72^\circ$

Answer

$72^\circ$

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