The Exterior Angle Theorem


If one side of a triangle is extended, an exterior angle is formed.  An exterior angle of a triangle forms a linear pair with the adjacent interior angle. The two non-adjacent interior angles to the exterior angle are called its remote interior angles. In the figure below, $latex \angle 1$ and $latex \angle 2$ are remote angles of $latex \angle 4$.

exterior angle

In the second triangle above, we can see that the sum of the measures of the remote angles is equal to the measure of the exterior angle. That is, $latex \angle 1 + \angle 2 = \angle 4$. Is this observation always true? In this post, we prove that it is indeed true


The measure of the exterior angle of a triangle is equal to the sum of the measure sf its interior angles.


$latex m \angle 3 + m \angle 4 = 180$ since the two angles are linear pair and therefore supplementary.

$latex m \angle 4 = 180 – m \angle 3$* by Addition Property of Equality

$latex m \angle 1 + m \angle 2 + m \angle 3 = 180$  because the angle sum of a triangle is $latex 180$ degrees.

$latex m \angle 1 + m \angle 2 = 180 – m \angle 3$ by Addition Property of Equality**

From * and **,  $latex m \angle 1 + m \angle 2$ and $latex m \angle 4$ both equal, $latex 180 – m \angle 3$,

$latex m \angle 1 + m \angle 2 = m \angle 4. \blacksquare$

3 thoughts on “The Exterior Angle Theorem

  1. Pingback: The Inscribed Angle Theorem - Proofs from The Book

  2. Pingback: Year in Review - The Most Popular Posts

  3. This should be proven in neutral geometry. You cannot assume triangles have angle sum 180 in neutral geometry. That’s Euclidean geometry only.

Leave a Reply

Your email address will not be published. Required fields are marked *