Before you check the box can you see how drawing a line through the top of the triangle parallel to the base will help to show that the sum of the three angles in a triangle is 180° ?

Directions:[br]1. Move the vertices of ∆ABC around.[br]2. Notice that the angles change when you move the vertices.[br]3. Pull the red slider to watch the animation.[br]4. What can you learn form this animation?