How do you know it is a outside angle

A triangle not only has inside angles but outside angles as well. When a triangle's side is extended, the extension creates another angle. This angle, called an outside or exterior angle, has a special relationship with the angle adjacent to it. The two angles become supplementary angles, which are angles that have a combined measurement of 180 degrees. You can find the measurements of a triangle's outside angles by working with the measurements of its inside ones.