Common Core State Standard: 8.G.5 

Grade  8 
Subject  MATH 
Domain  Geometry 
Theme  Understand congruence and similarity using physical models, transparencies, or geometry software 
Standard  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 angleangle criterion for similarity of triangles. For example, arrange three copies of the same triangle so that the three angles appear to form a line, and give an argument in terms of transversals why this is so. 
Reference URL  http://www.corestandards.org/Math/Content/8/G 