Lesson plan

11. Add side lengths to get perimeter (C)

teaches Common Core State Standards CCSS.Math.Content.3.MD.D.8 http://corestandards.org/Math/Content/3/MD/D/8
teaches Common Core State Standards CCSS.Math.Practice.MP8 http://corestandards.org/Math/Practice/MP8

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Lesson objective: Understand that perimeter is a measure of length calculated by adding the lengths of all of the sides of a polygon.

Students bring prior knowledge of addition strategies from 2.NBT.5. This prior knowledge is extended to calculating perimeter as students find efficient strategies to add side lengths. A conceptual challenge students may encounter is understanding that all side lengths need to be included in the sum, even if they are not labeled. Also, students often confuse perimeter with area.

The concept is developed through work with polygon models, which show different ways to think about perimeter, such as length calculated by laying the sides of a shape end to end or as the distance a bug would travel when walking around each side of a shape.

This work helps students deepen their understanding of operations because calculating perimeter can involve a variety of strategies, including those that involve addition and multiplication.

Students engage in Mathematical Practice 8 (Look for and express regularity in repeated reasoning) as they explore a variety of addition strategies and understand that, no matter what the shape of the polygon, perimeter is always calculated by adding the lengths of the sides in any order. 

Key vocabulary:

  • perimeter
  • polygon
  • sides