How can drawing a line through the top vertex of the triangle that is parallel to the base[br]help to show that the sum of the three angles in a triangle is 180°?

[b]Directions[/b]:[br]1. Move the vertices of ∆ABC around by dragging them.[br]2. Notice that the angles change when you move the vertices.[br]3. Click to place a check in the checkbox.[br]4. Pull the red slider to watch the animation.[br]5. Drag the vertices of ∆ABC again. What do you observe?[br][br]What do you know about the sum of the 3 angles at the top of the triangle?[br]What must the sum of the 3 angles of a triangle equal?[br][br]Click Reset to start over.

Move the sliders.[br]What do you notice about the exterior angle and the remote interior angles[br](the two angles [i]not[/i] adjacent to the exterior angle)?[br][br]Move the sliders back and drag the vertices to change the shape of the triangle to see if it always works.[br][br]What can you say about the sum of the two remote interior angles compared to the exterior angle?