# Grade 8 » Geometry » Understand congruence and similarity using physical models, transparencies, or geometry software. » 5

Print this pageUse 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*.