<img src="https://d5nxst8fruw4z.cloudfront.net/atrk.gif?account=iA1Pi1a8Dy00ym" style="display:none" height="1" width="1" alt="" />
Dismiss
Skip Navigation
Our Terms of Use (click here to view) have changed. By continuing to use this site, you are agreeing to our new Terms of Use.

6.6: Solutions to Compound Inequalities

Difficulty Level: At Grade Created by: CK-12
Atoms Practice
Estimated6 minsto complete
%
Progress
Practice Solutions to Compound Inequalities
Practice
Progress
Estimated6 minsto complete
%
Practice Now
Turn In

What if you had a compound inequality like 02x+66? How could you solve it and graph its solution set?

Solutions to Compound Inequalities

When we solve compound inequalities, we separate the inequalities and solve each of them separately. Then, we combine the solutions at the end.

 

 

 

 

 

 

 

 

Solve the following compound inequalities and graph the solution set.

a) 2<4x511

First we re-write the compound inequality as two separate inequalities with and. Then solve each inequality separately.

2<4x54x5113<4xand4x1634<x x4

 34<x and x4. This can be written as 34<x4.

b) 3x5<x+95x+13

 Re-write the compound inequality as two separate inequalities with and. Then solve each inequality separately.

3x5<x+9  x+95x+13  2x<14and44xx<7 1x

 x<7 and x1. This can be written as: 1x<7.

Solve the following compound inequalities and graph the solution set.

a) 92x3 or 3x+106x

Solve each inequality separately:

92x33x+106x 2x6or  4x4x3   x1

x3 or x1

b) x262x4 or x26>x+5

Solve each inequality separately:

x262x4x26>x+5x26(2x4)x2>6(x+5)x212x24or x2>6x+30 2211x 32>5x   2x6.4>x

x2 or x<6.4

One thing you may notice in the video for this Concept is that in the second problem, the two solutions joined with “or” overlap, and so the solution ends up being the set of all real numbers, or (,). This happens sometimes with compound inequalities that involve “or”; for example, if the solution to an inequality ended up being “x<5 or x>1,” the solution set would be all real numbers. This makes sense if you think about it: all real numbers are either a) less than 5, or b) greater than or equal to 5, and the ones that are greater than or equal to 5 are also greater than 1—so all real numbers are either a) less than 5 or b) greater than 1.

Compound inequalities with “and,” meanwhile, can turn out to have no solutions. For example, the inequality “x<3 and x>4” has no solutions: no number is both greater than 4 and less than 3. If we write it as 4<x<3 it’s even more obvious that it has no solutions; 4<x<3 implies that 4<3, which is false.

Solve Real-World Problems Using Compound Inequalities

Many application problems require the use of compound inequalities to find the solution.

Solve the Real-World Problem

The speed of a golf ball in the air is given by the formula v=32t+80. When is the ball traveling between 20 ft/sec and 30 ft/sec?

First we set up the inequality 20v30, and then replace v with the formula v=32t+80 to get 2032t+8030.

Then we separate the compound inequality and solve each separate inequality:

 2032t+80   32t+803032t60and 5032tt1.8751.56t

 1.56t1.875

To check the answer, we plug in the minimum and maximum values of t into the formula for the speed.

For t=1.56, v=32t+80=32(1.56)+80=30 ft/sec

For t=1.875, v=32t+80=32(1.875)+80=20 ft/sec

So the speed is between 20 and 30 ft/sec. The answer checks out.

Example

William’s pick-up truck gets between 18 to 22 miles per gallon of gasoline. His gas tank can hold 15 gallons of gasoline. If he drives at an average speed of 40 miles per hour, how much driving time does he get on a full tank of gas?

Let t= driving time. We can use dimensional analysis to get from time per tank to miles per gallon:

t hours1 tank×1 tank15 gallons×40 miles1 hour×40t15milesgallon

Since the truck gets between 18 and 22 miles/gallon, we set up the compound inequality 1840t1522. Then we separate the compound inequality and solve each inequality separately:

  1840t15 40t1522 27040tand40t3306.75t  t8.25

 6.75t8.25.

Andrew can drive between 6.75 and 8.25 hours on a full tank of gas.

If we plug in t=6.75 we get 40t15=40(6.75)15=18 miles per gallon.

If we plug in t=8.25 we get 40t15=40(8.25)15=22 miles per gallon.

The answer checks out.

Review

Solve the following compound inequalities and graph the solution on a number line.

  1. 5x413
  2. 13x+54
  3. \begin{align*}-12 \le 2-5x \le 7\end{align*}
  4. \begin{align*}\frac{3}{4} \le 2x+9 \le \frac{3}{2}\end{align*}
  5. \begin{align*}-2 \le \frac{2x-1}{3} < -1\end{align*}
  6. \begin{align*}4x-1 \ge 7\end{align*} or \begin{align*}\frac{9x}{2} < 3\end{align*}
  7. \begin{align*}3-x < -4\end{align*} or \begin{align*}3-x > 10\end{align*}
  8. \begin{align*}\frac{2x+3}{4} < 2\end{align*} or \begin{align*}-\frac{x}{5} + 3 < \frac{2}{5}\end{align*}
  9. \begin{align*}2x-7 \le -3\end{align*} or \begin{align*}2x-3 > 11\end{align*}
  10. \begin{align*}4x+3< 9\end{align*} or \begin{align*}-5x+4 \le -12\end{align*}
  11. How would you express the answer to problem 1 as a set?
  12. How would you express the answer to problem 1 as an interval?
  13. How would you express the answer to problem 6 as a set?
  14. Could you express the answer to problem 6 as a single interval? Why or why not?
    1. How would you express the first part of the solution in interval form?
    2. How would you express the second part of the solution in interval form?
  15. Express the answers to problems 2 through 5 in interval notation.
  16. Solve the inequality “\begin{align*}x \ge -3\end{align*} or \begin{align*}x < 1\end{align*}” and express the answer in interval notation.
  17. How many solutions does the inequality “\begin{align*}x \ge 2\end{align*} and \begin{align*}x \le 2\end{align*}” have?
  18. To get a grade of B in her Algebra class, Stacey must have an average grade greater than or equal to 80 and less than 90. She received the grades of 92, 78, 85 on her first three tests.
    1. Between which scores must her grade on the final test fall if she is to receive a grade of B for the class? (Assume all four tests are weighted the same.)
    2. What range of scores on the final test would give her an overall grade of C, if a C grade requires an average score greater than or equal to 70 and less than 80?
    3. If an A grade requires a score of at least 90, and the maximum score on a single test is 100, is it possible for her to get an A in this class? (Hint: look again at your answer to part a)

Review (Answers)

To see the Review answers, open this PDF file and look for section 6.6. 

Notes/Highlights Having trouble? Report an issue.

Color Highlighted Text Notes
Show More

Image Attributions

Show Hide Details
Description
Difficulty Level:
At Grade
Grades:
Date Created:
Aug 13, 2012
Last Modified:
Aug 16, 2016
Save or share your relevant files like activites, homework and worksheet.
To add resources, you must be the owner of the Modality. Click Customize to make your own copy.
Please wait...
Please wait...
Image Detail
Sizes: Medium | Original
 
MAT.ALG.354.L.1
Here