Exterior Angle
The exterior angle is the angle between any side of a shape and a line extended from the adjacent side.
An exterior angle is not the entire outside angle ... it is only the angle between one side and the line extended from the adjacent side.
We can extend either side and get the exact same angle:
More examples:
When we add up the interior angle and its matching exterior angle we get a straight angle 180°, so they are supplementary angles.
Exterior Angle + Interior Angle = 180°
Example: If the Interior Angle is 70°, what's the Exterior Angle?
We know they add up to 180°:
Exterior Angle = 180° − 70° = 110°