By the end of this section, you will be able to:
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 1degree below 0. A minus sign is shown before a number to indicate that it is negative. Figure 3.2 shows −20°F, which is 20degrees below 0.
Figure 3.2 Temperatures below zero are described by negative numbers.
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.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 1,302feet1,302feet below sea level, so the elevation of the Dead Sea can be represented as −1,302feet.−1,302feet. See Figure 3.3.
Figure 3.3 The surface of the Mediterranean Sea has an elevation of 0ft.0ft. The diagram shows that nearby mountains have higher (positive) elevations whereas the Dead Sea has a lower (negative) elevation.
Depths below the ocean surface are also described by negative numbers. A submarine, for example, might descend to a depth of 500feet.500feet. Its position would then be −500feet−500feet as labeled in Figure 3.4.
Figure 3.4 Depths below sea level are described by negative numbers. A submarine 500ft500ft below sea level is at −500ft.−500ft.
Both positive and negative numbers can be represented on a number line. Recall that the number line created in Add Whole Numbers started at 00 and showed the counting numbers increasing to the right as shown in Figure 3.5. The counting numbers (1, 2, 3, …)(1, 2, 3, …) on the number line are all positive. We could write a plus sign, +,+, before a positive number such as +2+2 or +3,+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 −1−1 at the first mark to the left of 0,−20,−2 at the next mark, and so on. See Figure 3.6.
Figure 3.6 On a number line, positive numbers are to the right of zero. Negative numbers are to the left of zero. What about zero? Zero is neither positive nor negative.
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.
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 Figure 3.10.
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 ab (read a is greater than b ) when a is to the right of b on the number line. This is shown for the numbers 3 and 5 in Figure 3.11.
Figure 3.11 The number 33 is to the left of 55 on the number line. So 33 is less than 5,5, and 55 is greater than 3.3.
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.
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 3.13(a). Similarly, 3 and −3 are opposites as shown in Figure 3.13(b).
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.
|10−4||Between two numbers, the symbol indicates the operation of subtraction.|
We read 10−4 as 10 minus 4 .
|−8||In front of a number, the symbol indicates a negative number.|
We read −8 as negative eight.
|−x||In front of a variable or a number, it indicates the opposite.|
We read −x as the opposite of x .
|−(−2)||Here we have two signs. The sign in the parentheses indicates that the number is negative 2.|
The sign outside the parentheses indicates the opposite. We read −(−2) as the opposite of −2.
The set of counting numbers, their opposites, and 00 is the set of integers.
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 Figure 3.16.
We treat absolute value bars just like we treat parentheses in the order of operations. We simplify the expression inside first.
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.
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.”
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.
Locate Positive and Negative Numbers on the Number Line
For the following exercises, draw a number line and locate and label the given points on that number line.
Order Positive and Negative Numbers on the Number Line
In the following exercises, order each of the following pairs of numbers, using < or >.
In the following exercises, find the opposite of each number.
In the following exercises, simplify.
In the following exercises, simplify each absolute value expression.
In the following exercises, evaluate each absolute value expression.
In the following exercises, fill in <,>,or= to compare each expression.
ⓑ −|−3|−3 30. ⓐ −8|−8|
ⓑ −|−2|−2 31. ⓐ |−3|−|−3|
ⓐ |−5|−|−5| ⓑ 9−|−9|
In the following exercises, simplify each expression.
Translate each phrase into an expression with integers. Do not simplify.
ⓐ the opposite of 8
ⓑ the opposite of −6
ⓒ negative three
ⓓ 4 minus negative 3
ⓐ the opposite of 11
ⓑ the opposite of −4
ⓒ negative nine
ⓓ 8 minus negative 2
ⓐ the opposite of 20
ⓑ the opposite of −5
ⓒ negative twelve
ⓓ 18 minus negative 7
ⓐ the opposite of 15
ⓑ the opposite of −9
ⓒ negative sixty
ⓓ 12 minus 5
Elevation The highest elevation in the United States is Mount McKinley, Alaska, at 20,320feet20,320feet above sea level. The lowest elevation is Death Valley, California, at 282feet282feet below sea level. Use integers to write the elevation of:
Extreme temperatures The highest recorded temperature on Earth is 58° Celsius,58° Celsius, recorded in the Sahara Desert in 1922. The lowest recorded temperature is 90°90° below 0° Celsius,0° Celsius, recorded in Antarctica in 1983. Use integers to write the:
State budgets In June, 2011, the state of Pennsylvania estimated it would have a budget surplus of $540 million.$540 million. That same month, Texas estimated it would have a budget deficit of $27 billion.$27 billion. Use integers to write the budget:
College enrollments Across the United States, community college enrollment grew by 1,400,0001,400,000 students from 20072007 to 2010.2010. In California, community college enrollment declined by 110,171110,171 students from 20092009 to 2010.2010. Use integers to write the change in enrollment:
Give an example of a negative number from your life experience.62.
What are the three uses of the “−” sign in algebra? Explain how they differ.
ⓐ After completing the exercises, use this checklist to evaluate your mastery of the objectives of this section.
ⓑ If most of your checks were:
…confidently. Congratulations! You have achieved the objectives in this section. Reflect on the study skills you used so that you can continue to use them. What did you do to become confident of your ability to do these things? Be specific.
…with some help. This must be addressed quickly because topics you do not master become potholes in your road to success. In math, every topic builds upon previous work. It is important to make sure you have a strong foundation before you move on. Whom can you ask for help? Your fellow classmates and instructor are good resources. Is there a place on campus where math tutors are available? Can your study skills be improved?
…no—I don’t get it! This is a warning sign and you must not ignore it. You should get help right away or you will quickly be overwhelmed. See your instructor as soon as you can to discuss your situation. Together you can come up with a plan to get you the help you need.