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.

  1. What do you observe about angles 1, 2 and 3
  2. What is the sum of angles 1, 2, and 3? Justify your answer. 
  3. Based on your observations above, what can you say about the sum of the interior angles of a triangle? Explain your answer. 

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