Timmy is trying out his new roller skates. He’s not allowed to cross the street yet, so he skates back and forth in front of his house. If he skates 20 yards east and then 10 yards west, how far is he from where he started? What if he skates 20 yards west and then 10 yards east?
The absolute value of a number is its distance from zero on a number line. There are always two numbers on the number line that are the same distance from zero. For instance, the numbers 4 and -4 are each a distance of 4 units away from zero.
represents the distance from 4 to zero, which equals 4.
represents the distance from -4 to zero, which also equals 4.
In fact, for any real number :
if is not negative, and if is negative.
Absolute value has no effect on a positive number, but changes a negative number into its positive inverse.
Evaluating Absolute Values
Evaluate the following absolute values.
Since 25 is a positive number, the absolute value does not change it.
Since -120 is a negative number, the absolute value makes it positive.
Since -3 is a negative number, the absolute value makes it positive.
Since 55 is a positive number, the absolute value does not change it.
Since is a negative number, the absolute value makes it positive.
Absolute value is very useful in finding the distance between two points on the number line. The distance between any two points and on the number line is or .
For example, the distance from 3 to -1 on the number line is .
We could have also found the distance by subtracting in the opposite order: . This makes sense because the distance is the same whether you are going from 3 to -1 or from -1 to 3.
Finding the Distance Between Two Points
Find the distance between the following points on the number line.
Distance is the absolute value of the difference between the two points.
a) 6 and 15
b) -5 and 8
c) -3 and -12
Remember: When we computed the change in and the change in as part of the slope computation, these values were positive or negative, depending on the direction of movement. In this discussion, “distance” means a positive distance only.
Solve an Absolute Value Equation
We now want to solve equations involving absolute values. Consider the following equation:
This means that the distance from the number to zero is 8. There are two numbers that satisfy this condition: 8 and -8.
When we solve absolute value equations we always consider two possibilities:
- The expression inside the absolute value sign is not negative.
- The expression inside the absolute value sign is negative.
Then we solve each equation separately.
Solve the following absolute value equations.
a) There are two possibilities: and
b) There are two possibilities: and
Find the distance between the values and on the number line.
The distance is the absolute value of the difference:
The distance between the two points is .
Evaluate the absolute values.
Find the distance between the points.
- 12 and -11
- 5 and 22
- -9 and -18
- -2 and 3
- -0.012 and 1.067
To view the Review answers, open this PDF file and look for section 6.7.