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Reason about angle sums, parallel lines cut by a transversal, and AA similarity.
Worksheets for this standard 1 topic
The standard, officially
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.