Lesson objective: Understand angle relationships formed from parallel lines and a transversal.

Students bring prior knowledge of angle relationships formed with two intersecting lines from 7.G.B.5 and transformations in 8.G.3. This prior knowledge is extended to two parallel lines intersected by a transversal as students determine that certain pairs of angles are congruent while other pairs are supplementary. A conceptual challenge students may encounter is identifying angles especially when there is more than one transversal.

The concept is developed as students investigate angle measures in a bookshelf to introduce the relationships formed with parallel lines and transversals. Students will use a protractor and rigid transformations to identify angle relationships.

This work helps students deepen their understanding of equivalence because students determine that pairs of angles have equal measures when they are alternate exterior, alternate interior, or corresponding angles. They also recognize when angle pairs are supplementary.

Students engage in Mathematical Practice 5 (use appropriate tools strategically) as they use a protractor as a tool to identify angle relationships. Using rigid transformations, students will create a model and make conjectures about angle relationships formed when a transversal crosses a set of parallel lines.

**Key vocabulary:**

- alternate exterior angles
- alternate interior angles
- corresponding angles
- exterior angles
- interior angles
- parallel lines
- transversal

**Special materials needed:**