Lesson objective: Find missing angles formed when a transversal crosses parallel lines.

This lesson helps to build procedural skill with writing equations to represent equivalent angle measures formed from congruent angle relationships. Equations are used here because they highlight students understanding of whether angles are congruent, supplementary, or neither. This work develops students' understanding that unique angle relationships are formed when lines intersect.

Students engage in Mathematical Practice 1 (Make sense of problems and persevere in solving them) as they explain correspondences between equations and diagrams of transversals crossing parallel lines.

**Key vocabulary:**

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

**Special materials needed:**