Shapes and Their Perimeter

Lesson 8

Math

Unit 5

3rd Grade

Lesson 8 of 16

Objective


Find perimeter by measuring side lengths in whole number units.

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

  • 2.MD.A.1
  • 3.OA.D.8

Criteria for Success


  1. Find the perimeter of a shape by measuring the length of each side and adding their lengths together.
  2. Write an equation to represent the perimeter of a shape (optionally including simplified equations, such as a multiplication equation for a regular polygon) (MP.2). 

Tips for Teachers


Lesson Materials

  • Ruler (1 per student) — These need to have both inch and centimeter measures. These will also be needed for the Problem Set and Homework.
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Anchor Tasks


Problem 1

Use your ruler to find the perimeter, in inches, of the following triangle.

Guiding Questions

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Problem 2

Use your ruler to find the perimeter, in centimeters, of the following octagon.

Guiding Questions

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Problem 3

Use your ruler to find the perimeter, in inches, of the following rectangle.

Guiding Questions

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Problem Set


Answer Keys

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Discussion of Problem Set

  • Which shape has the smallest perimeter in #1? How do you know?
  • What unit did you use to record the perimeters of the shapes in #1? Why?
  • What do you notice about the perimeters of the shapes in #1(b) and 1(e)?
  • I think that to find the perimeter of the shape in #2, I can find the perimeter of each triangle and add them together. Do you agree or disagree? Why? How does this problem challenge our understanding of perimeter?
  • Whose shape has more sides in #3? Do more sides mean a greater perimeter? Why or why not?
  • What multiplication equation can you use to represent the perimeter of the square in #4?
  • Explain to a partner how to use a ruler to find the perimeter of a shape.

Target Task


Find the perimeter, in centimeters, of the shape below. Show or explain your work. 

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 7

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Lesson 9

Lesson Map

A7CB09C2-D12F-4F55-80DB-37298FF0A765

Topic A: Attributes of Two-Dimensional Shapes

Topic B: Understanding Perimeter

Topic C: Distinguishing Between Area and Perimeter

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