Adding Fractions with Unlike Denominators

Contributor: Rachel Lewis. Lesson ID: 12777

Can you picture adding fractions with unlike denominators? Don't be "blue;" after you've "red" this lesson, you will learn to picture those fractions and easily add them in a fraction of the time!

categories

Elementary

subject
Math
learning style
Visual
personality style
Beaver
Grade Level
Intermediate (3-5)
Lesson Type
Skill Sharpener

Lesson Plan - Get It!

Audio:

You know that ½ + ½ = 1. What is ½ + ¼? Do you know how to add fractions with unlike denominators?

Fractions show part of a whole.

The top number is the numerator. The bottom number is the denominator. To add fractions, we have to look at the denominator first. If the denominator in the two fractions is the same number, we can simply add the numerators and keep the denominator the same.

What happens if the denominators of the two fractions are different?

Look at the two fractions below:

fraction diagram

Do you see how the blue and red sections are different sizes? We can’t add two fractions with different denominators. So, we have to find a common denominator to make them the same.

Take a moment to review how to add fractions with unlike denominators:

Find a common denominator.

In the model above, the first box is divided into two parts. The second box is divided into four parts.

We can use our picture to create like units for each fraction:

fraction diagram

Divide the first box into four parts like the second box. Divide the second box into two parts like the first box.

Now, we can count the shaded units.

Out of the eight parts in the first box, four parts are shaded. Our fraction is 48.

Out of the eight parts in the second box, two parts are shaded. Our fraction is 28.

Add the numerators.

4 + 2 = 6

Keep the denominator the same.

Our answer is 68.

Simplify. Six and eight can be divided by two, so we can simplify this fraction:

  6   → ÷ 2 →   3  
8 → ÷ 2 → 4

 

  1. Now, read an article on adding fractions from MathIsFun.com, Adding Fractions.
  2. Review adding fractions with like denominators in Example 1.
  3. Then, take a look at Example 2. How can you add 13 and 16? Try it on your own paper using our picture models from the example above.
  4. Share your answer with a parent or teacher.
  5. Read the explanation on the site to check your work. Did you find the correct answer?
  6. Now, try the pen and paper strategy lower on the web page. Press Play to watch the video. How is this strategy different from the picture form?
  7. Before you continue to Example 3, read "A Rhyme to Help You Remember." Recite the poem to a parent, teacher, or friend.
  8. Then, try Example 3. Check your work. If you need help finding common denominators, review the "Making the Denominators the Same" section after Example 3.

The denominator shows you the number of equal parts in a whole. You can’t add two fractions if they have different denominators. That’s why it is important to find the least common denominator before you add fractions together.

For more practice, go to the Got It? section to try games and activities!

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