Exterior Angle Theorem

The Exterior Angle Theorem states that the measure of an exterior angle of a triangle is equal to the sum of the measures of the two remote interior angles.

Remote interior angles are the two angles inside a triangle that are not adjacent to a given exterior angle.

The remote interior angles to Angle W are the two angles inside the triangle that do not share a vertex with Angle W.

To find the measure of an exterior angle, add the measures of the two remote interior angles.

The measure of the exterior angle is 50° + 70° = 120°.

The equation can be set up as: exterior angle = remote interior angle 1 + remote interior angle 2.

The measure of the exterior angle at Angle A is 40° + 60° = 100°.

The exterior angle is equal to the sum of the two remote interior angles, while the adjacent interior angle is not included in this sum.

The sum of the remote interior angles must equal 130°; for example, 70° and 60°.

By measuring the exterior angle and the two remote interior angles, and confirming that their sum equals the exterior angle.