Learning goals: Teacher Facing Create diagrams using 180-degree rotations of triangles to justify (orally and in writing) that the measure of angles in a triangle sum up to 180 degrees. Generalize the Triangle Sum Theorem using rigid transformations or the congruence of alternate interior angles of parallel lines cut by a transversal. Full lesson plan (warm-up, activities, synthesis, cool-down prompt) from 8.1 Rigid Transformations and Congruence, Grade 8.