Massachusetts 7 - 2020 Edition
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3.01 Simplifying algebraic expressions
Lesson

In a previous lesson we have looked at the different components of an expression, and how to construct algebraic expressions.

Recall that if we have one box containing $p$p apples, and then we get another box containing $p$p apples:

We can write $p$p apples plus $p$p more apples as:

Number of apples = $p+p$p+p

Remember that adding the same number multiple times is the same as multiplying it.
So two boxes of $p$p apples can be written as:

Number of apples = $p+p$p+p = $2p$2p

This is a very simple case of what is known as combining like terms. If we wanted to then add another $3$3 boxes of $p$p apples, that is we want to add $3p$3p to $2p$2p, we can see that we would have a total of $5p$5p apples.

$2p+3p$2p+3p $=$= $\left(p+p\right)+\left(p+p+p\right)$(p+p)+(p+p+p)
  $=$= $p+p+p+p+p$p+p+p+p+p
  $=$= $5p$5p

But what if we wanted to now add $4$4 boxes, each containing $q$q bananas to our existing boxes of apples?

$2p+3p+4q$2p+3p+4q $=$= $\left(p+p\right)+\left(p+p+p\right)+\left(q+q+q+q\right)$(p+p)+(p+p+p)+(q+q+q+q)
  $=$= $p+p+p+p+p+q+q+q+q$p+p+p+p+p+q+q+q+q
  $=$= $5p+4q$5p+4q

 

Can we simplify this addition any further?

We can not add $5$5 apples and $4$4 bananas into one combined term, because we wouldn't have $9$9 boxes of apples, nor would we have $9$9 boxes of bananas. What would we have? $9$9 Bapples? Bapples don't exist!

We can not simplify this expression any further, because $p$p and $q$q are not like terms. Replacing $p$p and $q$q with any other different variables and the same logic applies.

 

Definition: Like terms
Two algebraic terms are called like terms if they have exactly the same combination of variables.
This includes the exponents: $x$x and $x^2$x2 are not the same variables, in the same way that $4$4 and $4^2$42 are not equal.

 

Breaking it down

Let's look at the expression $9x+4y-5x+2y$9x+4y5x+2y. What does this mean, and how can we simplify it?

Remember that we leave out multiplication signs between numbers and variables. So we can read the expression as follows:

$9x$9x $+$+$4y$4y $-$$5x$5x $+$+$2y$2y
$9$9 groups of $x$x plus $4$4 groups of $y$y minus $5$5 groups of $x$x plus $2$2 groups of $y$y

Thinking about it this way, we can see that $9x$9x and $-5x$5x are like terms (they both represent groups of the same unknown value $x$x). We can now rearrange the equation, ensuring the sign attached the left of any term remains with it.

$9x$9x $-$$5x$5x $+$+$4y$4y $+$+$2y$2y
$9$9 groups of $x$x minus $5$5 groups of $x$x plus $4$4 groups of $y$y plus $2$2 groups of $y$y
  • If we have "$9$9 groups of $x$x" and subtract "$5$5 groups of $x$x", then we will be left with "$4$4 groups of $x$x". That is $9x-5x=4x$9x5x=4x.

  • Similarly, $4y$4y and $2y$2y are like terms, so we can add them: $4y+2y=6y$4y+2y=6y.

 

Putting this together, we have $9x+4y-5x+2y=4x+6y$9x+4y5x+2y=4x+6y.

Notice that we can't simplify $4x+6y$4x+6y any further. The variables $x$x and $y$y represent different unknown values, and they are not like terms.

 

Definition: Combine like terms

To combine like terms means to simplify an expression by combining all like terms together through addition and/or subtraction.

Worked example

Question 1

Simplify the following expression:

$3s+5t+2s+8t$3s+5t+2s+8t

Think: To simplify an expression we combine all the like terms. $3s$3s and $2s$2s both have the same variables so they are like terms and we can combine them. Similarly, $5t$5t and $8t$8t are also like terms.

Do: Let's rearrange the expression and group the like terms together so we can clearly see which terms we need to sum.

$3s+5t+2s+8t$3s+5t+2s+8t $=$= $3s+2s+5t+8t$3s+2s+5t+8t
  $=$= $5s+5t+8t$5s+5t+8t
  $=$= $5s+13t$5s+13t

Reflect: We identified like terms and then combined them until no like terms remained. We can add any of the terms together regardless of the ordering of the expression.

 

Practice questions

Question 2

Are the following like terms: $9y$9y and $10y$10y?

  1. Yes

    A

    No

    B

    Yes

    A

    No

    B

Question 3

Are the following like terms: $10x^2y$10x2y and $9y^2x$9y2x?

  1. Yes

    A

    No

    B

    Yes

    A

    No

    B

Question 4

Simplify the expression: $3c+4c+7c$3c+4c+7c

Outcomes

7.EE.1

Apply properties of operations to add, subtract, factor, and expand linear expressions with rational coefficients.

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