Angle Sum of a Triangle

1.) Move the vertices of the triangle to choose your own shape.
2.) Move point r1 to the extreme right of the slider to rotate ABC.
3.) Move point r2 to the extreme right of the slider to rotate A'B'C'.

1.) What can you say about the angles formed in the midpoint of AB''?
2.) How is this related to the interior angles of the original triangle?
3.) Make a conjecture about the sum of the interior angles of a triangle.

Guillermo P. Bautista Jr, December 17, 2009, Created with GeoGebra