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1.7.1: Simplify Variable Expressions Involving Addition and Subtraction

  • Page ID
    4312
  • Simplify Sums or Differences of Single Variable Expressions

    Figure \(\PageIndex{1}\)

    Marc is heading to his local ice cream store. This particular store doesn't have a lot of flavor choices, but they have the BEST vanilla ice cream around. Marc has taken orders from several of his neighbors, too, and has written them all down so that he can keep track of who wants what.

    The Johnsons - two vanilla ice cream cones

    The Mumfords - three vanilla ice cream cones

    Jill Stales - one vanilla ice cream cone

    The Porters - three vanilla ice cream cones

    How can Marc write this information as an algebraic expression and then simplify it?

    In this concept, you will learn how to simplify single variable expressions.

    Simplifying Sums or Differences of Single Variable Expressions

    If an expression has only numbers, you can calculate its numerical value. However, if an expression includes variables, it is helpful to simplify the expression.

    Look at this example.

    Simplify the expression 6a+3a.

    When adding expressions with variables, it is important to remember that only like terms can be combined. For example, 6a and 3a are like terms because both terms include the variable a. So, you can combine them.

    6a+3a=9a

    Here is another example.

    Simplify 6a+3.

    6a and 3 are not like terms because only one term includes the variable a. So, you cannot combine them. The expression 6a+3 cannot be simplified any further.

    Here is another example.

    Simplify 15d−2d.

    Since 15d and 2d both have the same variable, they are like terms. To find the difference, subtract the numerical parts of the terms the same way you would subtract any numbers.

    15d−2d=13d

    The answer is 13d.

    Here is an example using decimals.

    Simplify 0.4x+1.3x.

    Since 0.4x and 1.3x both have the same variable, they are like terms. To find the sum, add the numerical parts of the terms the same way you would add any decimals.

    0.4x+1.3x=1.7x

    The answer is 1.7x.

    Examples

    Example \(\PageIndex{1}\)

    Earlier, you were given a problem about Marc and all of his ice cream orders.

    Marc needs to write an expression to simplify his order: two vanillas, three vanillas, one vanilla, and three vanillas.

    Let v represent an ice cream cone. Then you can represent this situation as a single variable expression.

    Solution

    2v+3v+v+3v

    Looking at this expression, you will see that the variables are all the same. Therefore, simply add the numerical part of each term.

    2v+3v+v+3v=9v

    The answer is 9v.

    Example \(\PageIndex{1}\)

    Simplify the expression.

    5a+4a−2a+6a

    Solution

    To simplify this expression, follow the order of operations and combine like terms in order from left to right. Here is what the expression looks like after the first two terms have been combined.

    9a−2a+6a

    Next, perform the subtraction to get: 7a+6a

    Finally, add the terms.

    7a+6a=13a

    The answer is 13a.

    Simplify each sum or difference when possible.

    Example \(\PageIndex{1}\)

    3a+12a

    Solution

    These are like terms, so add the numerical parts together.

    The answer is 15a.

    Example \(\PageIndex{1}\)

    16x−2x

    Solution

    These are like terms, so subtract the numerical parts.

    The answer is 14x.

    Example \(\PageIndex{1}\)

    7y+2x

    Solution

    These are not like terms.

    The terms are not alike so you cannot combine them. The expression is in the simplest form already.

    The answer is 7y+2x.

    Review

    Simplify each sum or difference by combining like terms.

    1. 6a+7a
    2. 7x−2x
    3. 6y+12y
    4. 8a+12a
    5. 12y−7y
    6. 8a+15a
    7. 13b−9b
    8. 22x+19x
    9. 45y−12y
    10. 16a+18a+9a
    11. 14x−6x+2x
    12. 21a+14a−15a
    13. 33b+13b+8b
    14. 45x+67x−29x
    15. 92y+6y−54y

    Review (Answers)

    To see the Review answers, open this PDF file and look for section 7.4.

    Vocabulary

    Term Definition
    Difference The result of a subtraction operation is called a difference.
    Expression An expression is a mathematical phrase containing variables, operations and/or numbers. Expressions do not include comparative operators such as equal signs or inequality symbols.
    Simplify To simplify means to rewrite an expression to make it as "simple" as possible. You can simplify by removing parentheses, combining like terms, or reducing fractions.
    Sum The sum is the result after two or more amounts have been added together.

    Additional Resources

    Videos:

    PLIX: Play, Learn, Interact, eXplore: Algebraic Soup

    Practice: Simplify Variable Expressions Involving Addition and Subtraction

    Real World Application: Wages and Tips