# 7.4 Properties of Identity, Inverses, and Zero

The topics covered in this section are:

## 7.4.1 Recognize the Identity Properties of Addition and Multiplication

What happens when we add zero to any number? Adding zero doesn’t change the value. For this reason, we call $0$ the additive identity.

For example,

 $13+0$$13 -14+0$$-14$ $0+(-3x)$$-3x What happens when you multiply any number by one? Multiplying by one doesn’t change the value. So we call 11 the multiplicative identity. For example,  43 \cdot 1$$43$ $-27 \cdot 1$$-27 1 \cdot \frac{6y}{5}$$\frac{6y}{5}$

### IDENTITY PROPERTIES

The identity property of addition: for any real number $a$,

$a+0=a$ and $0+a=a$

$0$ is called the additive identity

The identity property of multiplication: for any real number $a$,

$a \cdot 1 = a$ and $1 \cdot a = a$

$1$ is called the multiplicative identity

#### Example 1

Identify whether each equation demonstrates the identity property of addition or multiplication.

• $7+0=7$
• $-16(1)=-16$
Solution

## 7.4.2 Use the Inverse Properties of Addition and Multiplication

 What number added to $5$ gives the additive identity, $0$? $5+$_____$=0$ We know $5+(-5)=0$ What number added to $-6$ gives the additive identity, $0$? $-6+$_____$=0$ We know $-6+6=0$

Notice that in each case, the missing number was the opposite of the number.

We call $-a$ the additive inverse of $a$. The opposite of a number is its additive inverse. A number and its opposite add to $0$, which is the additive identity.

What number multiplied by $\frac{2}{3}$ gives the multiplicative identity, $1$? In other words, two-thirds times what results in $1$?

 $\frac{2}{3} \cdot$_____$=1$ We know $\frac{2}{3} \cdot \frac{3}{2} = 1$

What number multiplied by $2$ gives the multiplicative identity, $1$? In other words two times what results in $1$?

 $2 \cdot$_____$=1$ We know $2 \cdot \frac{1}{2} = 1$

Notice that in each case, the missing number was the reciprocal of the number.

We call $\frac{1}{a}$ the multiplicative inverse of $a(a \neq 0)$. The reciprocal of a number is its multiplicative inverse. A number and its reciprocal multiply to $1$, which is the multiplicative identity.

We’ll formally state the Inverse Properties here:

### INVERSE PROPERTIES

Inverse Property of Addition for any real number $a$,

$a+(-a)=0$

$-a$ is the additive inverse of $a$.

Inverse Property of Multiplication for any real number $a \neq 0$,

$a \cdot \frac{1}{a} = 1$

$\frac{1}{a}$ is the multiplicative inverse of $a$.

#### Example 2

Find the additive inverse of each expression:

• $13$
• $- \frac{5}{8}$
• $0.6$
Solution

To find the additive inverse, we find the opposite.

• The additive inverse of $13$ is its opposite, $-13$.
• The additive inverse of $- \frac{5}{8}$ is its opposite, $\frac{5}{8}$.
• The additive inverse of $0.6$ is its opposite, $-0.6$.

#### Example 3

Find the multiplicative inverse:

• $9$
• $- \frac{1}{6}$
• $0.9$
Solution

To find the multiplicative inverse, we find the reciprocal.

• The multiplicative inverse of $9$ is its reciprocal, $\frac{1}{9}$.
• The multiplicative inverse of $- \frac{1}{9}$ is its reciprocal, $-9$.
• To find the multiplicative inverse of $0.9$, we first convert $0.9$ to a fraction, $\frac{9}{10}$. Then we find the reciprocal, $\frac{10}{9}$.

## 7.4.3 Use the Properties of Zero

We have already learned that zero is the additive identity, since it can be added to any number without changing the number’s identity. But zero also has some special properties when it comes to multiplication and division.

### Multiplication by Zero

What happens when you multiply a number by $0$? Multiplying by $0$ makes the product equal zero. The product of any real number and $0$ is $0$.

### MULTIPLICATION BY ZERO

For any real number $a$,

$a \cdot 0 = 0$ and $0 \cdot a = 0$

#### Example 4

Simplify:

• $-8 \cdot 0$
• $\frac{5}{12} \cdot 0$
• $0(2.94)$
Solution

### Dividing with Zero

What about dividing with $0$? Think about a real example: if there are no cookies in the cookie jar and three people want to share them, how many cookies would each person get? There are $0$ cookies to share, so each person gets $0$ cookies.

$0 \div 3 = 0$

Remember that we can always check division with the related multiplication fact. So, we know that

$0 \div 3 = 0$ because $0 \cdot 3 = 0$

### DIVISION OF ZERO

For any real number $a$, except $0$, $\frac{0}{a}=0$ and $0 \div a = 0$.

Zero divided by any real number except zero is zero.

#### Example 5

Simplify:

• $0 \div 5$
• $\frac{0}{-2}$
• $0 \div \frac{7}{8}$
Solution

Now let’s think about dividing a number by zero. What is the result of dividing $4$ by $0$? Think about the related multiplication fact. Is there a number that multiplied by $0$ gives $4$?

$4 \div 0 =$_____ means _____$\cdot 0 = 4$

Since any real number multiplied by $0$ equals $0$, there is no real number that can be multiplied by $0$ to obtain $4$. We can conclude that there is no answer to $4 \div 0$, and so we say that division by zero is undefined.

### DIVISION BY ZERO

For any real number $a, \frac{a}{0},$ and $a \div 0$ are undefined.

Division by zero is undefined.

#### Example 6

Simplify:

• $7.5 \div 0$
• $\frac{-32}{0}$
• $\frac{4}{9} \div 0$
Solution

We summarize the properties of zero.

### PROPERTIES OF ZERO

Multiplication by Zero: For any real number $a$,

$a \cdot 0 = 0$ and $0 \cdot a = 0$. The product of any number and $0$ is $0$.

Division by Zero: For any real number $a, a \neq 0$

$\frac{0}{a} = 0$. Zero divided by any real number, except itself, is zero.

$\frac{a}{0}$ is undefined.Division by zero is undefined.

## 7.4.4 Simplify Expressions using the Properties of Identities, Inverses, and Zero

We will now practice using the properties of identities, inverses, and zero to simplify expressions.

#### Example 7

Simplify: $3x+15-3x$.

Solution

#### Example 8

Simplify: $4(0.25q)$.

Solution

#### Example 9

Simplify: $\frac{0}{n+5}$, where $n \neq -5$.

Solution

#### Example 10

Simplify: $\frac{10-3p}{0}$.

Solution

#### Example 11

Simplify: $\frac{3}{4} \cdot \frac{4}{3} (6x+12)$.

Solution

We cannot combine the terms in parentheses, so we multiply the two fractions first.

All the properties of real numbers we have used in this chapter are summarized in Table 7.1.

Commutative Property
If $a$ and $b$ are real numbers then…$a+b=b+a$$a \cdot b = b \cdot a Associative Property If a,b, and c are real numbers then…(a+b)+c=a+(b+c)$$(a \cdot b) \cdot c = a \cdot ( b \cdot c )$
Identity Property$0$ is the additive identity$1$ is the multiplicative identity
For any real number $a$,$a+0=a$
$0+a=a$
$a \cdot 1 = a$
$1 \cdot a = a$
Inverse Property$-a$ is the additive inverse of $a$$a, a \neq 0 1/a is the multiplicative inverse of a For any real number a,a+(-a)=0$$a \cdot \frac{1}{a} = 1$
Distributive Property
If $a,b,c$ are real numbers, then $a(b+c)=ab+ac$
Properties of Zero
For any real number $a$,$a \cdot 0 = 0$
$0 \cdot a = 0$
For any real number $a, a \neq 0$$\frac{0}{a} = 0$
$\frac{a}{0}$ is undefined
Table 7.1 Properties of Real Numbers