# Lesson 9

Symmetry in Action (optional)

### Lesson Purpose

The purpose of this optional lesson is for students to practice visualizing and drawing figures given lines of symmetry and to use symmetry to solve problems about side lengths and perimeter.

### Lesson Narrative

In this lesson, students continue to work with line-symmetric figures in the context of paper folding. Students are given the result of folding paper along their lines of symmetry and reason about the original figure and its perimeter. As they think about the lengths of segments in line-symmetric shapes and find perimeters, students practice looking for and making use of structure (MP7).

This lesson is optional because the work here deepens and extends students’ understanding of line-symmetric figures and perimeter beyond what is required by the standards.

• Action and Expression
• MLR8

### Learning Goals

Teacher Facing

• Solve problems involving symmetry, side lengths, and perimeter of two-dimensional figures.

### Student Facing

• Let’s investigate symmetry and perimeter in folded figures.

### Required Materials

Materials to Gather

Materials to Copy

• Before and After

### 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

What was the best question you asked students today? Why would you consider it the best one based on what students said or did?