# 3.1 Introduction to Integers

The topics covered in this section are:

## 3.1.1 Locate Positive and Negative Numbers on the Number Line

Do you live in a place that has very cold winters? Have you ever experienced a temperature below zero? If so, you are already familiar with negative numbers. A negative number is a number that is less than $0$. Very cold temperatures are measured in degrees below zero and can be described by negative numbers. For example, $−1$°F (read as “negative one degree Fahrenheit”) is $1$ degree below $0$. A minus sign is shown before a number to indicate that it is negative. The figure below shows $−20$°F, which is $20$ degrees below $0$.

Temperatures are not the only negative numbers. A bank overdraft is another example of a negative number. If a person writes a check for more than he has in his account, his balance will be negative.

Elevations can also be represented by negative numbers. The elevation at sea level is $0$ feet. Elevations above sea level are positive and elevations below sea level are negative. The elevation of the Dead Sea, which borders Israel and Jordan, is about $1,302$ feet below sea level, so the elevation of the Dead Sea can be represented as $-1,302$ feet. See the figure below.

Depths below the ocean surface are also described by negative numbers. A submarine, for example, might descend to a depth of $500$ feet. Its position would then be $-500$ feet as labeled in the figure below.

Both positive and negative numbers can be represented on a number line. Recall that the number line created in Add Whole Numbers started at $0$ and showed the counting numbers increasing to the right as shown in the figure below. The counting numbers $(1,2,3,…)$ on the number line are all positive. We could write a plus sign, $+$ before a positive number such as $+2$ or $+3$ but it is customary to omit the plus sign and write only the number. If there is no sign, the number is assumed to be positive.

Now we need to extend the number line to include negative numbers. We mark several units to the left of zero, keeping the intervals the same width as those on the positive side. We label the marks with negative numbers, starting with $-1$ at the first mark to the left of $0,-2$ at the next mark, and so on. See the figure below.

The arrows at either end of the line indicate that the number line extends forever in each direction. There is no greatest positive number and there is no smallest negative number.

#### Example 1

Plot the numbers on a number line:

1. $3$
2. $-3$
3. $-2$
Solution

Draw a number line. Mark $0$ in the center and label several units to the left and right.

Part 1. To plot $3$, start at $0$ and count three units to the right. Place a point show in the figure below.

Part 2. To plot $-3$, start at $0$ and count three units to the left. Place a point as shown in the figure below.

Part 3. To plot $-2$, start at $0$ and count two units to the left. Place a point as shown in the figure below.

## 3.1.2 Order Positive and Negative Numbers

We can use the number line to compare and order positive and negative numbers. Going from left to right, numbers increase in value. Going from right to left, numbers decrease in value. See the figure below.

Just as we did with positive numbers, we can use inequality symbols to show the ordering of positive and negative numbers. Remember that we use the notation $a<b$ (read $a$ is less than $b$) when $a$ is to the left of $b$ on the number line. We write $a>b$ (read $a$ is greater than $b$) when $a$ is to the right of $b$ on the number line. This is show for the numbers $3$ and $5$ in the figure below.

The numbers lines to follow show a few more examples.

$4$ is to the right of $1$ on the number line, so $4>1$.

$1$ is to the left of $4$ on the number line, so $1<4$.

$-2$ is to the left of $1$ on the number line, so $-2<1$.

$1$ is to the right of $-2$ on the number line, so $1>-2$.

$-1$ is to the right of $-3$ on the number line, so $-1>-3$.

$-3$ is to the left of $-1$ on the number line, so $-3<-1$.

#### Example 2

Order each of the following pairs of numbers using $<$ or $>$:

1. $14$____$6$
2. $-1$____$9$
3. $-1$____$-4$
4. $2$____$-20$
Solution

Begin by plotting the numbers on a number line as show in the figure below.

## 3.1.3 Find Opposites

On the number line, the negative numbers are a mirror image of the positive numbers with zero in the middle. Because the numbers $2$ and $-2$ are the same distance from zero, they are called opposites. The opposite of $2$ is $-2$, and the opposite of $-2$ is $2$ as shown in Figure(a) below. Similarly, $3$ and $-3$ are opposites as show in Figure(b) below.

### OPPOSITE

The opposite of a number is the number that is the same distance from zero on the number line, but on the opposite side of zero.

#### Example 3

Find the opposite of each number:

1. $7$
2. $-10$
Solution

Part 1. The number $-7$ is the same distance from $0$ as $7$, but on the opposite side of $0$. So $-7$ is the opposite of $7$ as show in the figure below.

Part 2. The number $10$ is the same distance from $0$ as $-10$, but on the opposite side of $0$. So $10$ is the opposite of $-10$ as shown in the figure below.

#### Opposite Notation

Just as the same word in English can have different meanings, the same symbol in algebra can have different meanings. The specific meaning becomes clear by looking at how it is used. You have seen the symbol “$-$”, in three different ways.

### Opposite Notation

$-a$ means the opposite of the number $a$

The notation $-a$ is read the opposite of $a$.

#### Example 4

Simplify: $-(-6)$

Solution

#### Integers

The set of counting numbers, their opposites, and $0$ is the set of integers.

### Integers

Integers are counting numbers, their opposites, and zero.

$…-3,-2,-1,0,1,2,3…$

We must be very careful with the signs when evaluating the opposite of a variable.

#### Example 5

Evaluate $-x$:

1. when $x=8$.
2. when $x=-8$.
Solution

## 3.1.4 Simplify Expressions with Absolute Value

We saw that numbers such as $5$ and $-5$ are opposites because they are the same distance from $0$ on the number line. They are both five units from $0$. The distance between $0$ and any number on the number line is called the absolute value of that number. Because distance is never negative, the absolute value of any number is never negative.

The symbol for absolute value is two vertical lines on either side of a number. So the absolute value of $5$ is written as $|5|$, and the absolute value of $-5$ is written as $|-5|$ as shown in the figure below.

### Absolute Value

The absolute value of a number is its distance from $0$ on the number line.

The absolute value of a number $n$ is written as $|n|$.

$|n| \geq 0$ for all numbers

#### Example 6

Simplify:

1. $|3|$
2. $|-44|$
3. $|0|$
Solution

We treat absolute value bars just like we treat parentheses in the order of operations. We simplify the expression inside first.

#### Example 7

Evaluate:

1. $|x|$ when $x=-35$
2. $|-y|$ when $y=-20$
3. $-|u|$ when $u=12$
4. $-|p|$ when $p=-14$
Solution

Notice that the result is negative only when there is a negative sign outside the absolute value symbol.

Absolute value bars act like grouping symbols. First simplify inside the absolute value bars as much as possible. Then take the absolute value of the resulting number, and continue with any operations outside the absolute value symbols.

#### Example 8

Simplify:

1. $|9-3|$
2. $4|-2|$
Solution

For each expression, follow the order of operations. Begin inside the absolute value symbols just as with parentheses.

#### Example 9

Simplify: $|8+7|-|5+6|$.

Solution

For each expression, follow the order of operations. Begin inside the absolute value symbols just as with parentheses.

#### Example 10

Simplify: $24-|19-3(6-2)|$.

Solution

We use the order of operations. Remember to simplify grouping symbols first, so parentheses inside absolute value symbols would be first.

## 3.1.5 Translate Word Phrases into Expressions with Integers

Now we can translate word phrases into expressions with integers. Look for words that indicate a negative sign. For example, the word negative in “negative twenty” indicates $-20$. So does the word opposite in “the opposite of $20$.”

#### Example 11

Translate each prhase into an expression with integers:

1. the opposite of positive fourteen
2. the opposite of $-11$
3. negative sixteen
4. two minus negative seven
Solution
1. the opposite of positive fourteen
• $-14$
2. the opposite of $-11$
• $-(-11)=11$
3. negative sixteen
• $-16$
4. two minus negative seven
• $2-(-7)$

As we saw at the start of this section, negative numbers are needed to describe many real-world situations. We’ll look at some more applications of negative numbers in the next example.

#### Example 12

Translate into an expression with integers:

1. The temperature is $12$ degrees Fahrenheit below zero.
2. The football team had gain of $3$ yards.
3. The elevation of the Dead Sea is $1,302$ feet below sea level.
4. A checking account is overdrawn by $\$40\$.
Solution

Look for key phrases in each sentence. Then look for words that indicate negative signs. Don’t forget to include units of measurement described in the sentence.