exterior angle
an angle formed by one side of a polygon and the extension of an adjacent side
An exterior angle, also known as an external angle, is the angle formed when one side of a polygon is extended outward, and the adjacent interior angle is measured from the exterior point of intersection.
For example, in the figure of a triangle, if we extend one of its sides, say AB, beyond vertex B, we get an exterior angle ∠CBE. This exterior angle is formed by the extension of side AB and the adjacent interior angle ∠C of the triangle.
Exterior angles have some interesting properties. The measure of an exterior angle of a polygon is equal to the sum of its two adjacent interior angles. In other words, if we add the measures of two adjacent interior angles of a polygon, we get the measure of the exterior angle that is adjacent to them.
Thus, if we know the measures of the two interior angles adjacent to an exterior angle of a polygon, we can find the measure of the exterior angle by adding the measures of the two interior angles.
This property holds true for all polygons, whether they are regular or irregular. It is a useful property when we need to find a missing angle or verify the measures of angles in a polygon.
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