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أدلة الدراسة > Prealgebra

Rewriting Expressions Using the Commutative and Associative Properties

Learning Outcomes

  • Identify the associative and commutative properties of addition and multiplication
  • Use the associative and commutative properties of addition and multiplication to rewrite algebraic expressions
 

Use the Commutative and Associative Properties

Think about adding two numbers, such as 55 and 33. 5+33+588\begin{array}{cccc}\hfill 5+3\hfill & & & \hfill 3+5\hfill \\ \hfill 8\hfill & & & \hfill 8\hfill \end{array} The results are the same. 5+3=3+55+3=3+5 Notice, the order in which we add does not matter. The same is true when multiplying 55 and 33. 5⋅33⋅51515\begin{array}{cccc}\hfill 5\cdot 3\hfill & & & \hfill 3\cdot 5\hfill \\ \hfill 15\hfill & & & \hfill 15\hfill \end{array} Again, the results are the same! 5⋅3=3⋅55\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 aa and bb are real numbers, then a+b=b+aa+b=b+a Commutative Property of Multiplication: if aa and bb are real numbers, then a⋅b=b⋅aa\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.  

example

Use the commutative properties to rewrite the following expressions: 1. −1+3=-1+3= 2. 4⋅9=4\cdot 9= Solution:
1.
−1+3=-1+3=
Use the commutative property of addition to change the order. −1+3=3+(−1)-1+3=3+\left(-1\right)
2.
4⋅9=4\cdot 9=
Use the commutative property of multiplication to change the order. 4⋅9=9⋅44\cdot 9=9\cdot 4
 

try it

[ohm_question]145966[/ohm_question] [ohm_question]145968[/ohm_question]
  What about subtraction? Does order matter when we subtract numbers? Does 7−37 - 3 give the same result as 3−7?3 - 7? 7−33−74−44≠−4\begin{array}{ccc}\hfill 7 - 3\hfill & & \hfill 3 - 7\hfill \\ \hfill 4\hfill & & \hfill -4\hfill \\ & \hfill 4\ne -4\hfill & \end{array} The results are not the same. 7−3≠3−77 - 3\ne 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? 12÷44÷121244123133≠13\begin{array}{ccc}\hfill 12\div 4\hfill & & \hfill 4\div 12\hfill \\ \hfill \frac{12}{4}\hfill & & \hfill \frac{4}{12}\hfill \\ \hfill 3\hfill & & \hfill \frac{1}{3}\hfill \\ & \hfill 3\ne \frac{1}{3}\hfill & \end{array} The results are not the same. So 12÷4≠4÷1212\div 4\ne 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+27+8+2 How would you do it and what would your answer be? Some people would think 7+8 is 157+8\text{ is }15 and then 15+2 is 1715+2\text{ is }17. Others might start with 8+2 makes 108+2\text{ makes }10 and then 7+10 makes 177+10\text{ makes }17. Both ways give the same result, as shown below. (Remember that parentheses are grouping symbols that indicate which operations should be done first.) The image shows an equation. The left side of the equation shows the quantity 7 plus 8 in parentheses plus 2. The right side of the equation show 7 plus the quantity 8 plus 2. Each side of the equation is boxed separately in red. Each box has an arrow pointing from the box to the number 17 below. 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⋅13⋅35\cdot \frac{1}{3}\cdot 3 Changing the grouping of the numbers gives the same result. The image shows an equation. The left side of the equation shows the quantity 5 times 1 third in parentheses times 3. The right side of the equation show 5 times the quantity 1 third times 3. Each side of the equation is boxed separately in red. Each box has an arrow pointing from the box to the number 5 below. 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,ba,b, and cc are real numbers, then (a+b)+c=a+(b+c)\left(a+b\right)+c=a+\left(b+c\right) Associative Property of Multiplication: if a,ba,b, and cc are real numbers, then (a⋅b)⋅c=a⋅(b⋅c)\left(a\cdot b\right)\cdot c=a\cdot \left(b\cdot c\right)
 

example

Use the associative properties to rewrite the following: 1. (3+0.6)+0.4=\left(3+0.6\right)+0.4= 2. (−4⋅25)⋅15=\left(-4\cdot \frac{2}{5}\right)\cdot 15=

Answer: Solution:

1.
(3+0.6)+0.4=\left(3+0.6\right)+0.4=
Change the grouping. (3+0.6)+0.4=3+(0.6+0.4)\left(3+0.6\right)+0.4=3+\left(0.6+0.4\right)
Notice that 0.6+0.40.6+0.4 is 11, so the addition will be easier if we group as shown on the right.
2.
(−4⋅25)⋅15=\left(-4\cdot \frac{2}{5}\right)\cdot 15=
Change the grouping. (−4⋅25)⋅15=−4⋅(25⋅15)\left(-4\cdot \frac{2}{5}\right)\cdot 15=-4\cdot \left(\frac{2}{5}\cdot 15\right)
Notice that 25⋅15\frac{2}{5}\cdot 15 is 66. The multiplication will be easier if we group as shown on the right.

 

try it

[ohm_question]145970[/ohm_question] [ohm_question]145971[/ohm_question]
  Besides using the associative properties to make calculations easier, we will often use it to simplify expressions with variables.

example

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

Answer: Solution:

6(3x)6\left(3x\right)
Change the grouping. (6⋅3)x\left(6\cdot 3\right)x
Multiply in the parentheses. 18x18x
Notice that we can multiply 6⋅36\cdot 3, but we could not multiply 3⋅x3\cdot x without having a value for xx.

 

try it

[ohm_question]145973[/ohm_question]
The following video provides more examples of how to simplify expressions using the commutative and associative properties of multiplication and addition. https://youtu.be/UOcMDyJA7Yw

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