Lesson 9

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.

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.

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