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.
Try this 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

Triangle types

Triangle centers

Congruence and Similarity

Solving triangles

Triangle quizzes and exercises