Shapes and Their Perimeter

Lesson 11

Math

Unit 5

3rd Grade

Lesson 11 of 16

Objective


Find side lengths of regular polygons and of rectangles when given perimeter and other side lengths.

Common Core Standards


Core Standards

  • 3.MD.D.8 — Solve real world and mathematical problems involving perimeters of polygons, including finding the perimeter given the side lengths, finding an unknown side length, and exhibiting rectangles with the same perimeter and different areas or with the same area and different perimeters.

Foundational Standards

  • 3.OA.D.8

Criteria for Success


  1. Find the side length of a polygon when given an image of the figure, its perimeter, and all but one missing side length.
  2. Find the side length of a regular polygon when given an image of the figure and its perimeter.
  3. Find the side length of a rectangle when given an image of the figure, its perimeter, and one of its dimensions.
  4. Write an equation to represent the perimeter of a shape (including simplified equations, such as a multiplication equation for a regular polygon) (MP.2).

Tips for Teachers


This and the next lesson can be consolidated if you are tight on time. To do so, use Anchor Tasks 1 and 3 from this lesson and Anchor Task 1 from the next lesson. Then combine their Problem Sets, Target Tasks, and Homeworks to cover the content of both lessons equitably. 

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Anchor Tasks


Problem 1

The shape below has a perimeter of 48 meters.

What is the length, in meters, of the unknown side?

Guiding Questions

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Problem 2

The following shape is a regular pentagon. Its perimeter is 45 yards. What is the length of one side of the pentagon?

Guiding Questions

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Problem 3

The perimeter of the following rectangle is 56 centimeters. What is the rectangle’s length?

Guiding Questions

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Problem Set


Answer Keys

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Discussion of Problem Set

  • I think the width of the rectangle in #2 is 20 meters. Do you agree or disagree? Why? What mistake did I make?
  • How did you find the length of one side in #3? How did you solve?
  • How did you find the width of the rectangle in #5? Write an equation to represent how you solved. Is there more than one correct equation? 
  • Did anyone attempt #6? What fact did you need to use about rectilinear figures from Unit 4 to be able to solve?

Target Task


Problem 1

The shape below has a perimeter of 30 feet.

What is the length of the side that is missing a number?

Problem 2

Tessa ropes off a rectangular part of her lawn to create a play area for her dog. She uses 32 yards of rope. What is the missing side length of the play area?

Student Response

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Additional Practice


The Extra Practice Problems can be used as additional practice for homework, during an intervention block, etc. Daily Word Problems and Fluency Activities are aligned to the content of the unit but not necessarily to the lesson objective, therefore feel free to use them anytime during your school day.

Extra Practice Problems

Answer Keys

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Word Problems and Fluency Activities

Word Problems and Fluency Activities

Help students strengthen their application and fluency skills with daily word problem practice and content-aligned fluency activities.

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Lesson 10

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Lesson 12

Lesson Map

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Topic A: Attributes of Two-Dimensional Shapes

Topic B: Understanding Perimeter

Topic C: Distinguishing Between Area and Perimeter

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