Prove that the 3 interior angles of a triangle add to equal 180 degrees.

Drag the points of triangle ABC around and turn on the checkboxes to discover the relationship between the three interior angles of a triangle.
1. Add up the 3 angles of the triangle. What do they add up to equal?
2. Turn on the "draw auxiliary lines" checkbox. What is true about these lines?
3. Turn on the "show alternating interior angles" checkbox. What do you know about these types of angles?
4. What is the measure of a straight angle?
5. How does this applet help you prove the triangle sum theorem?