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Solving an absolute value inequality is similar to solving an absolute value equation,
with a few more considerations. When dealing with inequalities, you will be dealing with more possible values as solutions. Check out the following comparisons:
Now, let's formalize these observations into a more mathematical statement:
Absolute Value Inequalities:
If the symbol is < (or <): (and)
If a > 0, then the solutions to | x | < a
are x < a and x > -a.
Also written: -a < x < a |
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If a < 0, there is no solution to | x | < a. |
Think about it: absolute value is always positive (or zero), so, of course, it cannot be less than a negative number. |
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If the symbol is > (or >): (or)
If a > 0, then the solutions to | x | > a
are x > a or x < -a. |
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If a < 0, all real numbers will satisfy | x | > a. |
Think about it: absolute value is always positive (or zero), so, of course, it is greater than any negative number.. |
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For help with solving absolute value inequalities
on your calculator,
click here. |
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Keep in mind that your graphing calculator can be used to solve absolute value inequalities and/or double check your answers. |
Solve for x: | x - 3 | < 4 [Working with "less than or equal to"]
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and |
Case 2:
x - 3 > -4
x > -1 |
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Note that there are two parts to the solution and that the connecting word is "and". |
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Solution: x > -1 and x < 7
also written as: -1 < x < 7 |
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Solve for x: | x - 20 | > 5 [Working with "greater than"]
Case 1:
x - 20 > 5
x > 25 |
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or |
Case 2:
x - 20 < -5
x < 15 |
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Note that there are two parts to the solution and that the connecting word is "or". |
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Solution: x < 15 or x > 25 |
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Solve for x: | 3 + x | - 4 < 0 [Isolate absolute value.]
Case 1:
| 3 + x | < 4
3 + x < 4
x < 1 |
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or |
Case 2:
| 3 + x | < 4
3 + x > -4
x > -7 |
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Note that the absolute value is isolated before the solution begins. |
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Solution: x < 1 and x > -7
also written as:
-7 < x < 1
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Solve for x: 5 < | x + 1 | < 7 [compound inequality]
Separate a compound inequality into two separate problems. |
5 < | x + 1 | |
| x + 1 | < 7 |
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or |
Case 2:
-5 > x + 1
-6 > x |
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and |
Case 2:
x + 1 < -7
x > -8 |
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Solution: x > 4 or x < -6 |
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Now, find where the solutions overlap!
Solution: -8 < x < -6 as well as 4 < x < 6 |
Solve for x: | x + 4 | > -3 [All values work.]
Case 1:
x + 4 > -3
x > -7 |
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or |
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You already know the answer!
Absolute value is always positive (or zero),
so it is always > -3.
All values work! |
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Solution: x > -7 or x < -1
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Solve for x: | x + 1 | < -6 [No values work.]
Case 1:
x + 1 < -6
x < -7 |
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and |
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You already know the answer!
Absolute value is always positive (or zero).
It is NEVER < -6.
No values work! |
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Solution: x < -7 and x > 5 ??
The answer is the empty set Ø.
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[word problem]
It is reported that the average yearly salary for computer programmers in the United States is $51,423 per year, but can vary depending upon location. The actual salary could differ from the average by as much as $15,559 per year.
a) Write an absolute value inequality to describe this situation.
b) Solve the inequality to find the range of the starting salaries.
Solution:
Remember that | x - a | < b represents the set of all points that are less than b units
away from a.
a) | x - 51423 | < 15559
| the difference between the average and the salary | < $15,559
b)
Case 1:
| x - 51423 | < 15559
x - 51423 < 15559
x < 66982
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Case 2:
| x - 51423 | < 15559
x - 51423 > -15559
x > 35864
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Answer: $35,864 < x < $66,982
The absolute value inequality verifies what common sense tells you the answer to be. |
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