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Exterior Angle of a Triangle
See It in Action
Two examples of the idea done right—and one common look-alike that is not right.
EXAMPLE
EXAMPLE
NON-EXAMPLE
Definition
An exterior angle of a triangle is an angle formed when one side of the triangle is extended beyond a vertex.
Key points:
Each vertex of a triangle has two exterior angles (one on each side), but usually only one is considered at a time.
An exterior angle and its adjacent interior angle are a linear pair and add to 180°.
The Exterior Angle Theorem states that the measure of an exterior angle is equal to the sum of the two non-adjacent interior angles.
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