Solving absolute value inequalities


When solving absolute value inequalities, the process is very similar to solving absolute value equations. You should review the latter before studying this lesson.

Absolute value definition:

If x is positive, | x | = x

If x is negative, | x | = -x

Example #1:

Solve for x when | x | < 8

After applying the definition to example #1, you will have two equations to solve

In fact, when solving absolute value inequalities, you will usually get two solutions. That is important to keep in mind

If x is positive, | x | = x, so the first equation to solve is x < 8. Done because x is automatically isolated

If x is negative, | x | = -x, so the second equation to solve is -x < 8.

You can write -x < 8 as -1x < 8 and divide both sides by -1 to isolate x.

(-1/-1)x > 8/-1

1x > 8/-1

x > -8

Notice that the smaller sign (<) was switched to a bigger sign (>)

This happens whenever you divide or multiply inequalities by a negative number

Let us see why this makes sense

We know that 2 < 4. However, let us divide both sides by a negative number, say -2

2/-2 ? 4/-2

-1 ? -2

Should ? be < or >?

Since -1 is bigger than -2, it should be -1 > -2

So, the solutions are x < 8 and x > -8

x > -8 means the same thing as -8 < x

Putting -8 < x and x < 8 together, you can write -8 < x < 8

Therefore, any number between -8 and 8 is a solution

for instance, if we choose -5, we get | -5 | = 5 and 5 is smaller than 8

Example #2:

Solve for x when | x − 4 | < 7

Before, we apply the definition, let's make a useful substitution

Let y = x − 4, so | x − 4 | < 7 becomes | y | < 7. You must understand this step. No excuses!

Now, let's apply the definition to | y | < 7. Again, you will have two inequalities to solve

Once again, when solving absolute value inequalities, you will usually get two solutions.

If y is positive, | y | = y, so the first equation to solve is y < 7. No, you are not done! You have to substitute x − 4 for y

After substitution, y < 7 becomes x − 4 < 7

x − 4 < 7

x + -4 < 7

x + -4 + 4 < 7 + 4

x < 11

If y is negative, | y | = -y, so the second equation to solve is -y < 7.

You have to substitute x − 4 for y

You get -( x − 4) < 7. Notice the inclusion of parenthesis this time

-(x − 4) < 7

-(x + -4) < 7

-x + 4 < 7

-x + 4 − 4 < 7 − 4

-x < 3

(-1/-1)x > 3/-1

x > -3

The solutions are x > -3 and x < 11

Example #3:

Solve for x when | 3x + 3 | > 15

Before, we apply the definition, let's make a useful substitution

Let y = 3x + 3, so | 3x + 3 | > 15 becomes | y | > 15.

Now, let's apply the definition to | y | > 15.

Lastly, when solving absolute value inequalities, you will usually get two solutions. We may never say this enough!



If y is positive, | y | = y, so the first equation to solve is y > 15. You have to substitute 3x + 3 for y

After substitution, y > 7 becomes 3x + 3 > 15

3x + 3 > 15

3x + 3 − 3 > 15 − 3

3x > 12

(3/3)x > 12/3

x > 4

If y is negative, | y | = -y, so the second equation to solve is -y > 15.

After substitution, -y > 7 becomes -(3x + 3) > 15

-(3x + 3) > 15

-3x + -3 > 15

-3x + -3 + 3 > 15 + 3

-3x > 18

(-3/-3)x > 18/-3

x < -6

The solutions are x > 4 and x < -6

Solving absolute value inequalities should be straightforward if you follow my guidelines above





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