Animation that the sum of the angles of a triangle is a straight angle; α+β+γ=180°[br]Click and drag the the vertices of the triangle to change the angles!

How did we choose the segment for the reflection of point B? (Hint: Check the box for midpoints.)[br]How did we choose the segments for the reflections of point A and point C?[br]Which pairs of triangles are congruent (equal)?[br]How does this animation show that the sum of the angles of any triangle is 180°?