# Adding and Subtracting Unlike Fractions Lesson Plan: Check The Denominator!

In this lesson plan adaptable for grades 3-8, students use BrainPOP to learn how to identify the lowest common denominator in equations, and add and subtract unlike fractions. They’ll also explain in words and pictures how to add and subtract fractions.

### Students will:

- Identify the lowest common denominator in equations
- Add and subtract fractions with unlike denominators
- Explain in words and pictures how to add and subtract fractions

### Materials:

- Two small containers, bags, or baskets to hold the index cards during the activity
- One large sheet of unlined paper for each pair of students
- Computer and projector to show BrainPOP resources
- One index card for each student
- Scissors for students to cut apart index cards (or you can pre-cut them)

### Vocabulary:

fraction, one-half, numerator, denominator, common denominator, LCD (lowest common denominator)

### Preparation:

Teach an initial lesson to review adding fractions with like denominators and introduce the concept of LCD (lowest common denominator) to students. Make sure students understand how to find common multiples, and together, practice solving several problems that involve adding and subtracting fractions with unlike denominators.### Lesson Procedure:

- After your introductory lesson in adding and subtracting fractions with unlike denominators, activate student knowledge by beginning this lesson with the question on the FYI page: What's the easiest way to find a common denominator of two fractions? Give students time to recall and discuss this information with a partner, then share the FYI page with the class.
- Deepen students' understanding of the topic by showing the Adding and Subtracting Fractions movie.
- Pass out an index card to each student, and have them fold the cards in half. Each student should write a number sentence that involves either adding or subtracting fractions with unlike denominators, placing the answer portion of the equation on the second half of the index card. Ask students to show their equation to a partner to check for accuracy.
- Have students cut their index cards in half so the problem and answer to their equation are now on two separate pieces.
- Collect all the index card pieces from students, keeping the problems in one container and the answers in another.
- Divide the class in half, and randomly distribute the problems to one half of the class and the answers to other half.
- On your signal, have students stand up and push in their chairs, then walk over to the students in the group opposite of them to find the matching half of their equation. Students should then sit down next to each other to indicate they have found their match.
- Facilitate a discussion with the whole class regarding any remaining students who cannot find a match to ensure everyone has a correct equation. Is it possible that more than one problem might have the same answer?
- Have students return their index cards to the containers and go back to their seats, and then repeat the game play for an additional round or two.
- After the final round of play, have students keep their index cards and partners. Ask the pairs of students to find the lowest common denominator for the fractions on the card and write it on the back.
- Distribute a large sheet of unlined paper to each pair of students, and ask them to copy the equation from the index cards and the lowest common denominators for the equation onto the big paper.
- Students should then use the large paper to illustrate the fractions in a way that shows the relationship between the original equation and the equation that uses the lowest common denominator to make adding/subtracting possible. Students may wish to add instructions that show how to convert the numbers and solve.
- Put each pair of students with another pair to form a group of 4, and have them present their posters to the other two students in their group.