Education Technology

Adding and Subtracting Fractions with Common Denominators

Updated on 07/24/2015

Activity Overview

This activity uses interactive number lines to investigate addition and subtraction of fractions that refer to the same whole, with like denominators. Students may reason about the process as 1) decomposing each fraction into unit fractions and counting the total number of unit fractions that would be marked on the number line; or 2) the series of the lengths from 0 represented by each fraction, when those segments are joined end to end.

About the Lesson

This TI-Nspire lesson uses interactive number lines to investigate addition and subtraction of fractions that refer to the same whole, with like denominators. Students may reason about the process as 1) decomposing each fraction into unit fractions and counting the total number of unit fractions that would be marked on the number line; or 2) the series of the lengths from 0 represented by each fraction, when those segments are joined end to end. Subtraction can be thought of as finding the missing addend (i.e. for fraction-c-b- fraction-a-b, what would you add to fraction-a-b to obtain fraction-c-b?) or as taking away the number of copies of fraction-a-b from the number of copies of fraction-c-b.

Students should be familiar with the concept of fraction as a point on a number line before they begin this activity. Understanding fraction-a-b as a copies of fraction-1-b marked on the number line beginning from 0 suggests that adding two fractions with denominators of fraction-1-b is the same as joining the first segment fraction-a-b end to end with the second segment fraction-c-b. The activity can also be used to investigate the properties for addition (commutative, associative, inverse, and identity).

This is Lesson 5 out of 15 in the Building Concepts in Mathematics: Fractions Series.

Standards