5. 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.

In reference to the excerpt, “Use informal arguments to establish facts about the […] exterior angle of triangles” it sounds as though triangles have only one exterior angle.  Is that perhaps supposed to be the plural “exterior angles” or maybe “exterior angle sum” of which there is only one?

• This topic was modified 11 years, 3 months ago by Jim. Reason: Formatting