Equivalent Expressions

Contributor: Ashley Nail. Lesson ID: 13900

Everyone got different homework answers, but they're all correct! How is that possible? Use what you know about combining like terms and the distributive property to identify equivalent expressions.

categories

Expressions and Equations, Middle School, Pre-Algebra

subject
Math
learning style
Kinesthetic, Visual
personality style
Lion, Otter, Beaver, Golden Retriever
Grade Level
Middle School (6-8)
Lesson Type
Quick Query

Lesson Plan - Get It!

Audio:

Ailee, Liam, and Maddy are working on their homework together.

working on homework

On one problem, they all got different answers:

  Ailee Liam Maddy
  4x 3x + x 2(2x)

 

Their teacher tells them that each of their answers is correct.

  • How can that be true?

Ailee has the idea to plug a number value into each expression.

She replaces the variable x with 10:

  Ailee Liam Maddy
  4x 3x + x 2(2x)
  4 • 10 3 • 10 + 10 2(2 • 10)
  40 30 + 10 2 • 20
    40 40

 

All three expressions equal 40 when x = 10.

Maddy still doesn’t believe the expressions are equivalent.

She wants to plug 9 into each expression:

  Ailee Liam Maddy
  4x 3x + x 2(2x)
  4 • 9 3 • 9 + 9 2(2 • 9)
  36 27 + 9 2 • 18
    36 36

 

Again, the expressions all equal the same value when x = 9

Liam says you can prove the expressions are equivalent without even replacing the variable with a number. Let’s see if he’s correct.

First, let’s look at the expression:

3x + x

We can rewrite the expression as:

3x + 1x

Since both terms contain the variable x, they can be added together:

3x + 1x = 4x

Next, let’s look at the expression:

2(2x)

We can use the distributive property to multiply everything inside the parentheses by the factor outside of the parentheses.

We can rewrite the expression as:

2 • 2x

Now, we multiply:

2 • 2x = 4x

So all three students’ expressions are equivalent. No matter what number replaces the variable, the value of the expressions will all be equal.

Before you practice on your own, let’s look at one more example.

Which expressions below are equivalent to 2x + 15y + x + 9:

3(x + 5y + 3)

18xy + 9

3x + 15y + 9

3(x + y + 9)

First, let’s see if we can simplify the original expression.

  • Do you notice any like terms that can be combined?
  • 2x + 15y + x + 9
  • 2x + x + 15y + 9
  • 3x + 15y + 9

There are two terms that share the same variable x, so they can be combined.

You cannot combine the x and y terms together.

This shows us right away one equivalent expression and eliminates one expression from the list.

3(x + 5y + 3)

18xy + 9

3x + 15y + 9 

3(x + y + 9)

Now, let’s use distributive property to factor a number out of each term.

  • 2x + 15y + x + 9
  • 2x + x + 15y + 9
  • 3x + 15y + 9
  • (3 • 1)x + (3 • 5)y + (3 • 3)
  • 3(x + 5y +3)

Each term shares a factor of 3.

We can factor 3 out of each term and place it outside of the parentheses. This shows us the last equivalent expression!

3(x + 5y + 3)

18xy + 9

3x + 15y + 9 

3(x + y + 9)

Now you are ready to practice finding equivalent expressions on your own!

Click NEXT to visit the Got It? section.

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