Lesson plan

11. Determine the length of a missing side (C)

teaches Common Core State Standards CCSS.Math.Content.4.MD.A.3 http://corestandards.org/Math/Content/4/MD/A/3
teaches Common Core State Standards CCSS.Math.Practice.MP1 http://corestandards.org/Math/Practice/MP1
teaches Common Core State Standards CCSS.Math.Practice.MP8 http://corestandards.org/Math/Practice/MP8
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Lesson objective: Understand that we can use the area formula to determine the length of a missing side.

Students bring prior knowledge of determining the area of a rectangle by tiling it with unit squares or by multiplying the side lengths (3.MD.C.7.a). This prior knowledge is extended to solving for a missing side of a rectangle, as students are given the area and one side length. A conceptual challenge students may encounter is the understanding that the relationship between operations can be used to solve problems involving area. 

The concept is developed through work with rectangles and the area formula. Students represent these situations as both multiplication equations with a missing factor and division equations, which supports the inverse relationship between multiplication and division.

This work helps students deepen their understanding of operations that the inverse relationship between multiplication and division can be used to solve real world problems.

Students engage in Mathematical Practice 1 (make sense of problems and persevere in solving them) as they interpret area problems and look for efficient ways to solve them. Students engage in Mathematical Practice 8 (look for and express regularity in repeated reasoning) as they connect principles of operations to efficiently solve problems.

Key vocabulary:

  • inverse