# Measure of Exterior Angles

“What is the exterior angle and what is the interior angle?” This is a basic Geometry lesson that I hope will shed some light on this subject. For a complete summary please go to: MooMooMath

Measure of an Exterior Angle
Hi Welcome to MooMooMath. Today we are going to talk about the measure of the exterior angle of a triangle. We have a triangle here and we only know one angle. So first we have to decide “What is the exterior angle and what is the interior angle?” The interior angles are these three angles on the inside. To create an exterior I’m going to take the angle I know the 45 degrees and extend it out. Now I have created an exterior angle. So this is now an exterior angle associated with the 45 degree angle. Notice we have a straight line and we know that a straight line has a measure of 180 degrees. So these two are what we call linear pairs they are supplementary so I can find this exterior angle by taking 180 and subtract 45 so that equals 135 degrees. So let's look at the rules for exterior angles in a triangle. First you have to create the exterior angle by extending one side of the triangle. Then the measure is supplementary to the interior angle. Here is one more example. You have a triangle and the interior angle is 30 so we extend that side out to find the exterior angle so I subtract the 30 degrees from 180 which equals 150 degree angle. Hope this was helpful