# Lesson 15

### Lesson Purpose

The purpose of this lesson is to use known angles to reason about unknown angles and write equations to reason about unknown angles.

### Lesson Narrative

In this lesson, students use what they know about angles and their measurements to solve problems that are increasingly more complex and abstract. To find measurements of unknown angles, students need to look for structure in the diagrams, reason about the relationships of the angles (including writing equations to represent the relationships), and perform addition, subtraction, and sometimes division.

The problems in the lesson can be solved in more than one way and in different orders, but a small handful of the angles can only be quantified after the values of some other angles are known. Students pay attention to the process and explain why sometimes a certain sequence is necessary.

• Representation
• MLR8

### Learning Goals

Teacher Facing

• Represent angle relationships and solve for unknown angle measurements.

### Student Facing

• Let’s figure out missing angle measurements.

### Required Materials

Materials to Copy

• Info Gap: Whole Bunch of Angles

### Required Preparation

Activity 2:

• Create a set of cards from the blackline master for each group of 2.

### Lesson Timeline

 Warm-up 10 min Activity 1 15 min Activity 2 20 min Lesson Synthesis 10 min Cool-down 5 min

### Teacher Reflection Questions

As you finish up this unit, reflect on the norms and activities that have supported students’ development as thinkers. How have students grown in their ability to reason more abstractly and logically? What instructional strategies or routines worked well in supporting this development? What will you continue to do and improve on in Unit 7, and what will you adjust?

### Suggested Centers

• Which One? (K–5), Stage 4: Grade 3 Shapes (Supporting)
• Can You Draw It? (1–5), Stage 4: Area and Perimeter (Supporting)