G.CO.10 Prove theorems about triangles... measures of interior angles of a triangle sum to 180.
Students should be able to remember that the three angles that make up a triangle will always add up to 180. From this, students should be able to conclude why the triangle exterior angle theorem is true.
Consider the picture to the left for the proof. The dotted line (l) is parallel to the bottom segment (BC) of the triangle.
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Keywords: triangle angle sum theorem, triangle exterior angle theorem