Answer
1. The exterior angle is supplementary to the adjacent interior angle.
2. The sum of the angles of a triangle is 180 degrees.
3. Thus, the two nonadjacent interior angles are equal to 180 degrees minus the adjacent interior angles.
4. Thus, their sum can be thought of as supplementary to the adjacent interior angle.
5. If the sum of the nonadjacent interior angles as well as the exterior angle are supplementary to the adjacent interior angle, then the sum of the nonadjacent interior angles is equal to the exterior angle.
