Approach #2 The sum of the angles in Any Triangle

[list][br][*]Drag the slider to the end to rearrange the three parts of the triangle.[br][*]Move the slider back to its initial position.[br][*]Drag the vertices to change the shape of the rectangle and then drag the slider again.[br][/list][br]What is the sum of the angles of the triangle?
Describe what you notice?
How can these observations be used to prove that the sum of the angles in the triangle is 180 degrees?
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– Kim Johnson, Irina Boyadzhiev

Information: Approach #2 The sum of the angles in Any Triangle