Lesson objective: Understand that intersecting lines produce angles that have important relationships.

Students bring prior knowledge of drawing and identifying angles as right, acute, or obtuse, from 4.G.A.1. This prior knowledge is extended to recognizing how these angles are related as students classify the angles as vertical, complementary, or supplementary. A conceptual challenge students may encounter is the idea that complementary angles add to equal 90 degrees and supplementary angles add to equal 180 degrees.

The concept is developed through work with a diagram, which asks students to identify pairs of angles that are congruent, pairs of angles that add to equal a right angle, and pairs of angles that add to equal a straight angle.

This work helps students deepen their understanding of operations because the relationships among angles allow students to find the measures of unknown angles in a geometric figure.

Students engage in Mathematical Practice 7 (look for and make use of structure) as they notice the angles formed by intersecting lines and begin to classify them as vertical angles, complementary angles, and supplementary angles.

**Key vocabulary:**

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

**Special materials needed:**