# 7. Understand how equivalent expressions can help us solve subtraction problems

teaches Common Core State Standards CCSS.Math.Content.2.OA.A.1 http://corestandards.org/Math/Content/2/OA/A/1
teaches Common Core State Standards CCSS.Math.Practice.MP8 http://corestandards.org/Math/Practice/MP8

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Lesson objective: Understand that creating an equivalent expression can be used as a strategy to solve a subtraction problem.

Students bring prior knowledge of addition and subtraction from 2.OA.A.1. This prior knowledge is extended to get comfortable with even more addition and subtraction strategies as students create equivalent expressions as a way to solve subtraction problems. A conceptual challenge students may encounter is not making an equivalent expression, resulting in an incorrect answer.

The concept is developed through work with a number line, which is a clear visual of how students are adding and subtracting.

This work helps students deepen their understanding of equivalence because using an equivalent expression to solve an addition or subtraction problem will not change the total or difference.

Students engage in Mathematical Practice 8 (Look for and express regularity in repeated reasoning) as they try to convince a student that equivalent expressions can be used as a strategy to add or subtract.

Key vocabulary:

• difference
• equivalent
• expression
• sum

Special materials needed:

• cuisenaire rods
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