Lesson objective: Find the measures of unknown angles in geometric figures using relationships that exist among complementary, supplementary, vertical, and adjacent angles.

This lesson helps to build procedural skill with these special angle relationships. Visual representations are used to support students as they determine angle relationships and find unknown angle measures. This work develops students' understanding that equations can be used to represent the missing angle measure based on whether the angles are complementary, supplementary, vertical, or adjacent.

Students engage in Mathematical Practice #7 (look for and make use of structure) as they use the structure of intersecting lines to look at the various angles that are formed by the lines. Students engage in productive struggle while writing multi-step equations that represent these special angle relationships.

**Key vocabulary:**

- acute angle
- adjacent angle
- complementary angle
- non-adjacent angle
- obtuse angle
- right angle
- straight angle
- supplementary angle
- vertical angle

**Special materials needed:**