# Lesson 7

Ways to Find Unknown Length (Part 1)

### Lesson Purpose

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

### Lesson Narrative

In previous lessons, students examined the attributes of two-dimensional figures. They identified, sorted, and drew figures based on side length, angle size, presence of parallel or perpendicular sides, and symmetry. In this lesson, students use their understanding of these attributes to find the perimeter of different shapes. In the first activity, students find the perimeter of shapes when all side lengths are given and connect the perimeter of different shapes to equations. In the second activity, students are asked to find the perimeter of shapes when some side lengths are not given. Students use what they have learned about analyzing the attributes of shapes to determine if the perimeters can be found or if they need more information.

• Representation
• MLR8

### Learning Goals

Teacher Facing

• Find the perimeter of two-dimensional shapes using their properties.

### Student Facing

• Let’s find the perimeter of different shapes.

### Required Materials

Materials to Gather

Building On

### Lesson Timeline

 Warm-up 10 min Activity 1 20 min Activity 2 15 min Activity 3 10 min Lesson Synthesis 10 min Cool-down 5 min

### Teacher Reflection Questions

Earlier in the unit, students looked closely at the attributes of two-dimensional figures. In what ways did you see students applying their understanding to find perimeter? How can you continue to foster connections between different geometric ideas in upcoming lessons?