2.6 Solve Compound Inequalities

Learning objectives

Learning objectives

• Solve compound inequalities with “and”

• Solve compound inequalities with “or”

• Solve applications with compound inequalities

Solve Compound Inequalities with “and”

Now that we know how to solve linear inequalities, the next step is to look at compound inequalities. A compound inequality is made up of two inequalities connected by the word “and” or the word “or.” For example, the following are compound inequalities.

To solve a compound inequality means to find all values of the variable that make the compound inequality a true statement. We solve compound inequalities using the same techniques we used to solve linear inequalities. We solve each inequality separately and then consider the two solutions.

To solve a compound inequality with the word “and,” we look for all numbers that make both inequalities true. To solve a compound inequality with the word “or,” we look for all numbers that make either inequality true.

Let’s start with the compound inequalities with “and.” Our solution will be the numbers that are solutions to both inequalities known as the intersection of the two inequalities. Consider the intersection of two streets—the part where the streets overlap—belongs to both streets.

To find the solution of the compound inequality, we look at the graphs of each inequality and then find the numbers that belong to both graphs—where the graphs overlap.

For the compound inequality $x>-3$ and $x\le{}2,$ we graph each inequality. We then look for where the graphs “overlap”. The numbers that are shaded on both graphs, will be shaded on the graph of the solution of the compound inequality. See Figure 2.5.

We can see that the numbers between $-3$ and $2$ are shaded on both of the first two graphs. They will then be shaded on the solution graph.

The number $-3$ is not shaded on the first graph and so since it is not shaded on both graphs, it is not included on the solution graph.

The number two is shaded on both the first and second graphs. Therefore, it is be shaded on the solution graph.

This is how we will show our solution in the next examples.

Example 2.61

Solution

Example 2.62

Solution

Example 2.63

Solution

Sometimes we have a compound inequality that can be written more concisely. For example, $a<x$ and $x<b$ can be written simply as $a<x<b$ and then we call it a double inequality. The two forms are equivalent.

To solve a double inequality we perform the same operation on all three “parts” of the double inequality with the goal of isolating the variable in the center.

Example 2.64

Solution

When written as a double inequality, $1\le{}x<5,$ it is easy to see that the solutions are the numbers caught between one and five, including one, but not five. We can then graph the solution immediately as we did above.

Another way to graph the solution of $1\le{}x<5$ is to graph both the solution of $x\ge{}1$ and the solution of $x<5.$ We would then find the numbers that make both inequalities true as we did in previous examples.

Solve Compound Inequalities with “or”

To solve a compound inequality with “or”, we start out just as we did with the compound inequalities with “and”—we solve the two inequalities. Then we find all the numbers that make either inequality true.

Just as the United States is the union of all of the 50 states, the solution will be the union of all the numbers that make either inequality true. To find the solution of the compound inequality, we look at the graphs of each inequality, find the numbers that belong to either graph and put all those numbers together.

To write the solution in interval notation, we will often use the union symbol, $\cup$ to show the union of the solutions shown in the graphs.

Example 2.65

Solution

Example 2.66

Solution

Solve Applications with Compound Inequalities

Situations in the real world also involve compound inequalities. We will use the same problem solving strategy that we used to solve linear equation and inequality applications.

Recall the problem solving strategies are to first read the problem and make sure all the words are understood. Then, identify what we are looking for and assign a variable to represent it. Next, restate the problem in one sentence to make it easy to translate into a compound inequality. Last, we will solve the compound inequality.

Example 2.67

Solution

Section 2.6 Exercises

Section 2.6 Exercises

Solve Compound Inequalities with “and”

In the following exercises, solve each inequality, graph the solution, and write the solution in interval notation.

Solve Compound Inequalities with “or”

In the following exercises, solve each inequality, graph the solution on the number line, and write the solution in interval notation.

Mixed practice

In the following exercises, solve each inequality, graph the solution on the number line, and write the solution in interval notation.

Solve Applications with Compound Inequalities

In the following exercises, solve.

Everyday Math

Writing Exercises

ⓐ After completing the exercises, use this checklist to evaluate your mastery of the objectives of this section.

ⓑ What does this checklist tell you about your mastery of this section? What steps will you take to improve?

1. Solve the compound inequality. Graph the solution and write the solution in interval notation: $4x-7<9$ and $5x+8\ge{}3.$

2. Solve the compound inequality. Graph the solution and write the solution in interval notation: $3x-4<5$ and $4x+9\ge{}1.$

3. Solve the compound inequality. Graph the solution and write the solution in interval notation: $2(3x+1)\le{}20$ and $4(x-1)<2.$

1. Solve the compound inequality. Graph the solution and write the solution in interval notation: $5(3x-1)\le{}10$ and $4(x+3)<8.$

2. Solve the compound inequality. Graph the solution and write the solution in interval notation: $\frac{1}{4}x-3\ge-1$ and $-3(x-2)\ge{}2.$

3. Solve the compound inequality. Graph the solution and write the solution in interval notation: $\frac{1}{5}x-5\ge-3$ and $-4(x-1)\ge-2.$

1. Solve the compound inequality. Graph the solution and write the solution in interval notation: $-5\le{}4x-1<7.$

2. Solve the compound inequality. Graph the solution and write the solution in interval notation: $-3<2x-5\le{}1.$

3. Solve the compound inequality. Graph the solution and write the solution in interval notation: $1-2x\le-3$ or $7+3x\le{}4.$

1. Solve the compound inequality. Graph the solution and write the solution in interval notation: $2-5x\le-3$ or $5+2x\le{}3.$

2. Solve the compound inequality. Graph the solution and write the solution in interval notation: $\frac{3}{5}x-7\le-1$ or $\frac{1}{3}(x+6)\ge-2.$

3. Solve the compound inequality. Graph the solution and write the solution in interval notation: $\frac{3}{4}x-3\le{}3$ or $\frac{2}{5}(x+10)\ge{}0.$

1. Due to the drought in California, many communities now have tiered water rates. There are different rates for Conservation Usage, Normal Usage and Excessive Usage. The usage is measured in the number of hundred cubic feet (hcf) the property owner uses.

During the summer, a property owner will pay $24.72 plus$1.32 per hcf for Conservation Usage. The bill for Conservation Usage would be between or equal to $31.32 and$52.12. How many hcf can the owner use if she wants her usage to stay in the conservation range?

2. Due to the drought in California, many communities have tiered water rates. There are different rates for Conservation Usage, Normal Usage and Excessive Usage. The usage is measured in the number of hundred cubic feet (hcf) the property owner uses.

During the winter, a property owner will pay $24.72 plus$1.54 per hcf for Normal Usage. The bill for Normal Usage would be between or equal to $49.36 and$86.32. How many hcf will he be allowed to use if he wants his usage to stay in the normal range?

3. $x\le{}4$ and $x>-2$

1. $x>-6$ and $x<-3$

2. $4x-1<7$ and $2x+8\ge{}4$

3. $4x-2\le{}4$ and$7x-1>-8$

1. $7x-8<6$ and$5x+7>-3$

2. $5(3x-2)\le{}5$ and$3(x+3)<3$

3. $-3(x+4)<0$ and$-1(3x-1)\le{}7$

1. $\frac{3}{4}(x-8)\le{}3$ and$\frac{1}{5}(x-5)\le{}3$

2. $\frac{3}{4}x-5\ge-2$ and$-3(x+1)\ge{}6$

3. $\frac{1}{2}(x-6)+2<-5$ and$4-\frac{2}{3}x<6$

1. $-3<2x-5\le{}1$

2. $-1<3x+2<8$

3. $-6\le{}4x-2<-2$

1. $x\le-4$ or $x>-3$

2. $x<0$ or $x\ge{}4$

3. $4-3x\le-2$ or$2x-1\le-5$

1. $3(2x-3)<-5$ or$4x-1>3$

2. $\frac{2}{3}x-3>5$ or $3(5-x)>6$

3. $2(x+3)\ge{}0$ or$3(x+4)\le{}6$

1. $\frac{3}{4}x+2\le-1$ or$\frac{1}{2}(x+8)\ge-3$

2. $6(2x-1)>6$ and$5(x+2)\ge{}0$

3. $\frac{1}{2}x-5\le{}3$ or$\frac{1}{4}(x-8)\ge-3$

1. $\frac{1}{5}(x-5)+6<4$ and$3-\frac{2}{3}x<5$

2. $6x-3\le{}1$ and$5x-1>-6$

3. $-5\le{}3x-2\le{}4$

1. Gregory is thinking of a number and he wants his sister Lauren to guess the number. His first clue is that six less than twice his number is between four and forty-two. Write a compound inequality that shows the range of numbers that Gregory might be thinking of.

2. Elouise is creating a rectangular garden in her back yard. The length of the garden is 12 feet. The perimeter of the garden must be at least 36 feet and no more than 48 feet. Use a compound inequality to find the range of values for the width of the garden.

3. Body Mass Index (BMI) is a measure of body fat is determined using your height and weight.

ⓐ Let x be your BMI. Research and then write the compound inequality to show the BMI range for you to be considered normal weight.

ⓑ Research a BMI calculator and determine your BMI. Is it a solution to the inequality in part (a)?

1. Explain the steps for solving the compound inequality $2-7x\ge-5$ or $4(x-3)+7>3.$