This section uses a geometric interactive to explore the Triangle Exterior Angle Property.

An exterior angle is the angle formed when one side is extended beyond its adjacent sides.

The red angles in the diagram show two different exterior angles, one to the left and one to the right of each interior angle of the triangle.

In the page linked below, the sketch displays one exterior angle for ΔABC. Click on and drag the orange dot at vertex A to change the value of vertex A and the exterior angle.

Exterior Angles of Triangles

While dragging vertex A around the screen, observe the exterior angle and the interior angles.

Write your observations and answers to the following questions questions in your notes.

• Notice the equation above the diagram, where are the values coming from?
• As vertex A increases, what is the value of the exterior angle doing?
• As vertex A decreases, what is the value of the exterior angle?
• What is the value of vertex B and the exterior angle?
• What types of angles are vertex B and the exterior angle?
• Review the questions above while dragging vertex B around the screen.

After experimenting with the diagram, click on the two correct statements/conjectures below.

Correct answers will remain bold.

Review the correct statements. Write a statement describing your conclusion.

Interactive popup. Assistance may be required. Check Your Answer Triangle Exterior Angle Property: The measure of an exterior angle of a triangle is equal to the sum of the measures of the remote interior angles.