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Linear Inequalities

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Linear Inequalities

What if you had an inequality with an unknown variable like x - 12 > -5 ? How could you isolate the variable to find its value? After completing this Concept, you'll be able to solve one-step inequalities like this one.

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CK-12 Foundation: 0602S Solving One-Step Inequalities (H264)

Guidance

To solve an inequality we must isolate the variable on one side of the inequality sign. To isolate the variable, we use the same basic techniques used in solving equations.

We can solve some inequalities by adding or subtracting a constant from one side of the inequality.

Example A

Solve the inequality and graph the solution set.

x-3<10

Solution

Starting inequality: x-3 < 10

Add 3 to both sides of the inequality: x - 3 + 3 < 10 + 3

Simplify: x < 13

Example B

Solve the inequality and graph the solution set.

x-20 \le 14

Solution:

Starting inequality: x - 20 \le 14

Add 20 to both sides of the inequality: x - 20 + 20 \le 14 + 20

Simplify: x \le 34

Solving Inequalities Using Multiplication and Division

We can also solve inequalities by multiplying or dividing both sides by a constant. For example, to solve the inequality 5x<3 , we would divide both sides by 5 to get x < \frac{3}{5} .

However, something different happens when we multiply or divide by a negative number. We know, for example, that 5 is greater than 3. But if we multiply both sides of the inequality 5>3 by -2, we get -10 > -6 . And we know that’s not true; -10 is less than -6.

This happens whenever we multiply or divide an inequality by a negative number, and so we have to flip the sign around to make the inequality true. For example, to multiply 2 < 4 by -3, first we multiply the 2 and the 4 each by -3, and then we change the < sign to a > sign, so we end up with -6 > -12 .

The same principle applies when the inequality contains variables.

Example C

Solve the inequality.

4x < 24

Solution:

Original problem: 4x < 24

Divide both sides by 4: \frac{4x}{4} < \frac{24}{4}

Simplify: x < 6

Example D

Solve the inequality.

-5x \le 21

Solution:

Original problem: -5x \le 21

Divide both sides by -5 : \frac{-5x}{-5} \ge \frac{21}{-5} Flip the inequality sign.

Simplify: x \ge -\frac{21}{5}

Watch this video for help with the Examples above.

CK-12 Foundation: Solving One-Step Inequalities

Vocabulary

  • The answer to an inequality is usually an interval of values .
  • Solving inequalities works just like solving an equation. To solve, we isolate the variable on one side of the equation.
  • When multiplying or dividing both sides of an inequality by a negative number, you need to reverse the inequality .

Guided Practice

Solve each inequality.

a) x+8 \le -7

b) x+4 > 13

c) \frac{x}{25} < \frac{3}{2}

d) \frac{x}{-7} \ge 9

Solutions:

a) Starting inequality: x+8 \le -7

Subtract 8 from both sides of the inequality: x + 8 - 8 \le -7 - 8

Simplify: x\le -15

b) Starting inequality: x+4 > 13

Subtract 4 from both sides of the inequality: x + 4 - 4 > 13 - 4

Simplify: x > 9

c) Original problem: \frac{x}{25} < \frac{3}{2}

Multiply both sides by 25: 25 \cdot \frac{x}{25} < \frac{3}{2} \cdot 25

Simplify: x < \frac{75}{2} or x < 37.5

d) Original problem: \frac{x}{-7} \ge 9

Multiply both sides by -7: -7 \cdot \frac{x}{-7} \le 9 \cdot (-7) Flip the inequality sign.

Simplify: x \le -63

Practice

For 1-8, solve each inequality and graph the solution on the number line.

  1. x-5 < 35
  2. x+15 \ge -60
  3. x-2 \le 1
  4. x-8 > -20
  5. x+11>13
  6. x+65<100
  7. x-32 \le 0
  8. x+68 \ge 75

For 9-11, solve each inequality. Write the solution as an inequality and graph it.

  1. 3x \le 6
  2. \frac{x}{5} > -\frac{3}{10}
  3. -10x>250
  4. \frac{x}{-7} \ge -5

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