Connecting a solid understanding of the unit fraction with prior knowledge of whole number addition and subtraction will lead to more powerful fraction sense and accurate fraction computation.
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See the standards throughout the grades: http://www.corestandards.org/Math/Content/NF/
NF3.c. Understand a fraction a/b (where a is a whole number) as a sum of fractions 1/b. Understand addition and subtraction of fractions as joining and separating parts referring to the same whole. Add and subtract fractions (including mixed numbers) by replacing given fractions with equivalent fractions to produce an equivalent sum or difference of fractions with like denominators. Solve word problems involving the addition and subtraction of fractions referring to the same whole.
NF2.a. Use concrete (set, area and length models) and pictorial (set, area and length models) to represent problems involving the addition, subtraction, multiplication, or division of fractions. Compare and contrast the different models.
NF2.b. Recognize common student errors involving fraction operations. (Involving addition and subtraction)
The focus of these topics should emphasize reasoning and sense-making about the procedures for addition and subtraction of fractions. Students should develop, connect, and represent addition and subtraction fraction procedures using manipulatives, number line models, and bar diagrams.
Making Sense Series - Fractions: The Meaning, Equivalence, & Comparison Video (Graham Fletcher)
Beyond Pizzas & Pies: 10 Essential Strategies for Supporting Fraction Sense Book (Julie McNamara and Meghan Shaughnessy)
Beyond Invert & Multiply: Making Sense of Fraction Computation Book (Julie McNamara)
Extending Children's Mathematics: Fractions and Decimals Book (Susan Empson and Linda Levi)
Uncomplicating Fractions to Meet Common Core Standards in Math, K-7 Book (Marian Small)
"The role of representations in fraction addition and subtraction." Article in Mathematics Teaching in the Middle School, vol. 13, issue 8, pgs. 490-496.
Beckmann (2018) Resources:
Chapter 3
Activity 3O Why Do We Add and Subtract Fractions the Way We Do?
Activity 3P Critiquing Mixed Number Addition and Subtraction Methods
Activity 3Q Are These Word Problems for 1/2 + 1/3?
Activity 3R Are These Word Problems for 1/2 - 1/3?
Billstein (2016) Resources:
Section 6-2
Sowder (2017) Resources:
Chapter 7
Activity 1 Can You Picture This?
Activity 2 Other Ways to Think of Subtraction
Van De Walle (2019) Resources:
Chapter 15
Contextual Examples
Models
Estimation