# Lesson 8

Ways to Find Unknown Length (Part 2)

### Lesson Purpose

The purpose of this lesson is for students to use the attributes of two-dimensional figures (such as perimeter and symmetry) to reason about side lengths.

### Lesson Narrative

In the previous lesson, students used given side lengths and symmetry to find the perimeter of a figure. In this lesson, students reason in the other direction—given the perimeter and information about symmetry, they find the side lengths of two-dimensional figures. Students also practice completing a figure given a line of symmetry and half of the figure, and then reason about the perimeter of the whole figure. Along the way, students reinforce their ability to add fractions and to multiply fractions by whole numbers.

• Engagement
• MLR8

Activity 2: Lin’s Design

### Learning Goals

Teacher Facing

• Find the unknown side lengths of two-dimensional shapes using their attributes.

### Student Facing

• Let’s find the unknown lengths in figures.

### Required Materials

Materials to Gather

### Lesson Timeline

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

### Teacher Reflection Questions

The tasks in this lesson prompted students to apply what they learned about adding fractions and multiplying fractions by a whole number. How comfortable were students with these operations? What ideas or strategies do students need more practice with?