Triangle Angle Sum Theorem (V4)

How it works ?

1. In the applet below, note the triangle and its 3 large white vertices.
2. You can move the vertices anywhere you’d like at any time.
3. The pink slider controls the size of the (soon-to-appear) pink angle.
4. Interact with this resource for a few minutes. As you do, be sure to move the vertices of the triangle around each time before re-sliding the slider.
5. Please answer the question that follows.