Angle Sum Theorem
Move points A, B, and C to choose the triangle size and shape you want. Move sliders 
and 
to the extreme right.
- What do you observe about angles 1, 2 and 3?
- What is the sum of angles 1, 2, and 3? Justify your answer.
- Based on your observations above, what can you say about the sum of the interior angles of a triangle? Explain your answer.