Exterior Angles of Triangles
An exterior (or external) angle is the angle between one side of a triangle and the extension of an adjacent side.
Drag the orange dots on any vertex
to reshape the triangle. One of the six possible exterior angles is shown. Note how it changes.
An exterior angle of a triangle is equal to the sum of the opposite interior angles.
For more on this see Triangle external angle theorem.
If the equivalent angle is taken at each vertex, the exterior angles always add to 360°
In fact, this is true for any convex polygon, not just triangles. See
Exterior angles of a polygon.
In the figure above, drag the triangle's vertices and see that this is so.
Other triangle topics
Perimeter / Area
Congruence and Similarity
Triangle quizzes and exercises
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