1.9: Properties of Real Number Addition
On the first day of school, you are all dressed in your new clothes. When you got dressed, you put one sock on your left foot and one sock on your right foot. Would it have made a difference if you had put one sock on your right foot first and then one sock on your left foot?
Guidance
There are five properties of addition that are important for you to know.
Commutative Property
In algebra, the operation of addition is commutative. The order in which you add two real numbers does not change the result, as shown below:
The order in which you added the numbers did not affect the answer. This is called the commutative property of addition. In general, the commutative property of addition states that the order in which two numbers are added does not affect the sum. If
Closure Property
The sum of any two real numbers will result in a real number. This is known as the closure property of addition. The result will always be a real number. In general, the closure property states that the sum of any two real numbers is a unique real number. If
Associative Property
The order in which three or more real numbers are grouped for addition will not affect the sum. This is known as the associative property of addition. The result will always be the same real number. In general, the associative property states that order in which the numbers are grouped for addition does not change the sum. If
Additive Identity
If zero is added to any real number the answer is always the real number. Zero is known as the additive identity or the identity element of addition. The sum of a number and zero is the number. This is called the identity property of addition. If
Additive Inverse
The sum of any real number and its additive inverse is zero. You learned in example D that zero is also called the identity element of addition. Therefore, it can be said that the sum of any real number and its additive inverse is the identity element. This is called the inverse property of addition. If
Example A
Use a number line to show that
On a number line, you add a positive number by moving to the right on the number line and you add a negative number by moving to the left on the number line.
Example B
Does
The result is 8. This is an integer. An integer is a real number. This is an example of the closure property.
Example C
Does
The numbers in the problem were the same but on the left side of the equal sign, the numbers 4 and +7 were grouped in parenthesis. The addition in the parenthesis was completed first and then +5 was added to that sum. The final result was +8.
On the right side of the equal sign, the numbers +7 and +5 were grouped in parenthesis. The addition in the parenthesis was completed first and then (4) was added to the sum. The final result was +8. This is an example of the associative property of addition.
Example D
Does
Example E
Does
When any real number is added to its opposite, the result is always zero. If
Concept Problem Revisited
Think back to the question about putting on socks. The order in which you put on the socks does not affect the outcome – you have one sock on each foot.
This is like the commutative property in algebra. The order in which you add two real numbers does not change the result.
Vocabulary
 Additive Identity
 The additive identity for addition of real numbers is zero.
 Additive Inverse

The additive inverse of addition is the opposite of the real number and the sum of the real number and its additive inverse is zero. If
a is any real number, its additive inverse is−a .
 Associative Property

The associative property of addition states the order in which three or more real numbers are grouped for addition, will not affect the sum. If
a,b andc are real numbers, then(a+b)+c=a+(b+c) .
 Closure Property

The closure property of addition states that the sum of any two real numbers is a unique real number. If
a,b andc are real numbers, thena+b=c .
 Commutative Property

The commutative property of addition states that the order in which two numbers are added, does not affect the sum. If
a andb are real numbers, thena+b=b+a .
 Identity Element of Addition
 The identity element of addition is another term for the additive identity of addition. Therefore, the identity element of addition is zero.
 Identity Property

The identity property of addition states that the sum of a number and zero is the number. If
a is a real number, thena+0=a .
 Inverse Property

The inverse property of addition states that the sum of any real number and its additive inverse is zero. If
a is a real number, thena+(−a)=0 .
Guided Practice
1. Add using the properties of addition:
2. What property justifies the statement?
3. Apply the commutative property of addition to the following problem.
Answers:
1.
2. \begin{align*}(21+6)+8=21+(6+8)\end{align*}
The numbers on each side of the equal sign are the same but they are not grouped the same.
\begin{align*}&(21+6)+8 && 21+(6+8)\\ &=(15)+8 && =21+(14)\\ &=7 && =7\end{align*}
The order in which the numbers were grouped did not affect the answer. The property that is being used is the associative property of addition.
3.
\begin{align*}& 17x15y\\ & 17x+(15y) \ \mathbf{Rewrite \ the \ statement \ as \ an \ addition \ statement.}\\ & 17x+(15y)=15y+17x\end{align*}
The commutative property of addition states that the order in which two numbers are added, does not affect the sum.
Practice
Match the following addition statements with the correct property of addition.
1. \begin{align*}(5)+5=0\end{align*}
2. \begin{align*}(16+4)+5=16+(4+5)\end{align*}
3. \begin{align*}9+(7)=16\end{align*}
4. \begin{align*}45+0=45\end{align*}
5. \begin{align*}9+(6)=(6)+9\end{align*}
a) Commutative Property
b) Closure Property
c) Inverse Property
d) Identity Property
e) Associative Property
Add the following using the properties of addition:
 \begin{align*}24+(18)+12\end{align*}
 \begin{align*}21+34+21\end{align*}
 \begin{align*}5+\left(\frac{2}{5}\right)+\left(\frac{3}{5}\right)\end{align*}
 \begin{align*}19+(7)+9\end{align*}
 \begin{align*}8+\frac{3}{7}+\left(\frac{3}{7}\right)\end{align*}
Name the property of addition that is being shown in each of the following addition statements:
 \begin{align*}(12+7)+10=12+(7+10)\end{align*}
 \begin{align*}18+0=18\end{align*}
 \begin{align*}16.5+18.4=18.4+16.5\end{align*}
 \begin{align*}52+(75)=23\end{align*}
 \begin{align*}(26)+(26)=0\end{align*}
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Here you will learn the properties of addition that apply to real numbers. These properties are the commutative property, the closure property, the associative property, the identity property and the inverse property. You will learn what is meant by each of these properties and how each one applies to the addition of real numbers. You will learn to apply the properties to addition, how to recognize the property as it is applied to addition and to name the property that justifies a given addition statement.
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