Commutative and Associative Properties

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Course: RWM101: Foundations of Real World Math
Book: Commutative and Associative Properties
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Date: Saturday, April 20, 2024, 6:37 PM

Description

Read this section and do the problems to practice using the commutative property. Complete the practice questions and check your answers.

Commutative and Associative Properties

In the next few sections, we will take a look at the properties of real numbers. Many of these properties will describe things you already know, but it will help to give names to the properties and define them formally. This way we'll be able to refer to them and use them as we solve equations in the next chapter.

Use the Commutative and Associative Properties

Think about adding two numbers, such as 5 and 3 .

\begin{array}{cc} 5+3 & 3+5 \\ 8 & 8 \end{array}

The results are the same. 5+3=3+5
Notice, the order in which we add does not matter. The same is true when multiplying 5 and 3.

 \begin{array}{cc} 5 \cdot 3 & 3 \cdot 5 \\ 15 & 15 \end{array}

Again, the results are the same! 5 \cdot 3=3 \cdot 5. The order in which we multiply does not matter. These examples illustrate the commutative properties of addition and multiplication.


COMMUTATIVE PROPERTIES
Commutative Property of Addition: if a and b are real numbers, then

 a+b=b+a

Commutative Property of Multiplication: if a and b are real numbers, then

 a \cdot b=b \cdot a

The commutative properties have to do with order. If you change the order of the numbers when adding or multiplying, the result is the same.

What about subtraction? Does order matter when we subtract numbers? Does 7-3 give the same result as 3-7?


    \begin{array}{llc}
    7-3 & & 3-7 \\
    4 & & -4 \\
    & 4 \neq-4 &
    \end{array}


\text { The results are not the same.} 7−3 ≠ 3−7

Since changing the order of the subtraction did not give the same result, we can say that subtraction is not commutative.

Let's see what happens when we divide two numbers. Is division commutative? 


            \begin{array}{ccc}
            12 \div 4 & & 4 \div 12 \\
            \dfrac{12}{4} & & \dfrac{4}{12} \\
            3 & & \dfrac{1}{3} \\
            & 3 \neq \dfrac{1}{3} &
            \end{array}



            \text { The results are not the same. So } 12 \div 4 \neq 4 \div 12

 

Since changing the order of the division did not give the same result, division is not commutative.

Addition and multiplication are commutative. Subtraction and division are not commutative.

Suppose you were asked to simplify this expression.

7+8+2

How would you do it and what would your answer be?

Some people would think 7+8 is 15 and then 15+2 is 17. Others might start with 8+2 makes 10 and then 7+10 makes 17.

Both ways give the same result, as shown in Figure 7.3. (Remember that parentheses are grouping symbols that indicate which operations should be done first).


Figure 7.3

When adding three numbers, changing the grouping of the numbers does not change the result. This is known as the Associative Property of Addition.

The same principle holds true for multiplication as well. Suppose we want to find the value of the following expression:

5 \cdot \frac{1}{3} \cdot 3

Changing the grouping of the numbers gives the same result, as shown in Figure 7.4.

f7.4

Figure 7.4.

When multiplying three numbers, changing the grouping of the numbers does not change the result. This is known as the Associative Property of Multiplication.

If we multiply three numbers, changing the grouping does not affect the product.

You probably know this, but the terminology may be new to you. These examples illustrate the Associative Properties.



ASSOCIATIVE PROPERTIES

Associative Property of Addition: if a, b, and c are real numbers, then

(a+b)+c=a+(b+c)

Associative Property of Multiplication: if a, b, and c are real numbers, then

(a \cdot b) \cdot c=a \cdot(b \cdot c)

Besides using the associative properties to make calculations easier, we will often use it to simplify expressions with variables.


Evaluate Expressions using the Commutative and Associative Properties

The commutative and associative properties can make it easier to evaluate some algebraic expressions. Since order does not matter when adding or multiplying three or more terms, we can rearrange and re-group terms to make our work easier, as the next several examples illustrate.



Source: Rice University, https://openstax.org/books/prealgebra/pages/7-2-commutative-and-associative-properties
Creative Commons License This work is licensed under a Creative Commons Attribution 4.0 License.

Examples and Exercises

EXAMPLE 7.5

Use the commutative properties to rewrite the following expressions:

(a) -1+3=\ _________

(b) 4 \cdot 9= _________


EXAMPLE 7.6

Use the associative properties to rewrite the following:

(a) (3+0.6)+0.4= ________

(b) \left(-4 \cdot \frac{2}{5}\right) \cdot 15= ________


EXAMPLE 7.7

Use the Associative Property of Multiplication to simplify: 6(3 x).

 

TRY IT 7.9

Use the commutative properties to rewrite the following:
(a) -4+7= ______
(b) 6 \cdot 12= ______
 

 

TRY IT 7.10

Use the commutative properties to rewrite the following:
(a) 14+(-2)= ______
(b) 3(-5)= ______

 

Answers

Solution to Example 7.5

 
  -1+3 = \text{______}
Use the commutative property of addition to change the order. -1+3 = 3+(-1)

 

 
  4 \cdot 9 = \text{______}
Use the commutative property of multiplication to change the order. 4 \cdot 9 = 9 \cdot 4


Solution to Example 7.6

 
  (3+0.6)+0.4=______
Change the grouping.

(3+0.6)+0.4=3+(0.6+0.4)______


 
  \left(-4 \cdot \frac{2}{5}\right) \cdot 15= _______
Change the grouping. \left(-4 \cdot \frac{2}{5}\right) \cdot 15=-4 \cdot\left(\frac{2}{5} \cdot 15\right)

 

Solution to Example 7.7

  6(3 x)
Change the grouping. (6 \cdot 3) x
Multiply in the parentheses. 18 x


Try It 7.9

(a) -4+7=7+(-4)

(b) 6 \cdot 12=12 \cdot 6


Try It 7.10

(a) 14+(-2)=-2+14

(b) 3(-5)=(-5) 3