Exterior angle

For a regular polygon, find one exterior angle using $360° \div n$; an interior angle is $180°$ minus that exterior angle.

If an exterior angle is given, use $360° \div \text{exterior angle}$ to find the number of sides.

State the geometric reason beside each step, such as “angles on a straight line add to $180^\circ$” or “exterior angles of a polygon add to $360^\circ$”.