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Use informal arguments to establish facts about the angle sum and exterior angle of triangles, about the angles created when parallel lines are cut by a transversal, and the angle-angle criterion for similarity of triangles. For example, arrange three copies of the same triangle so that the sum of the three angles appears to form a line, and give an argument in terms of transversals why this is so.


Find the Missing Angle
Find the Angle
Tile Patterns I: octagons and squares
Tile Patterns II: hexagons
Rigid motions and congruent angles
A Triangle's Interior Angles
Congruence of Alternate Interior Angles via Rotations
Street Intersections
Similar Triangles II
Similar Triangles I