Instructional Activity# Alternate Interior Angles Are Congruent

Lesson 14. Unit 1. Grade 8 Open Up Resources

Open Up Resources15 min(s)

This Instructional Activity is a part of the Lesson 14, Unit 1, Grade 8. The goal of this task is to experiment with rigid motions to help visualize why alternate interior angles (made by a transversal connecting two parallel lines) are congruent. This result will be used in a future lesson to establish that the sum of the angles in a triangle is 180 degrees. The second question is optional if time allows. This provides a deeper understanding of the relationship between the angles made by a pair of (not necessarily parallel) lines cut by a transversal. Expect informal arguments as students are only beginning to develop a formal understanding of parallel lines and rigid motions.