Write the solution as a compound inequality

In both cases, the solutions of compound inequalities can be expressed as graphs on the number line and also as interval notations.

I suggest that you first graph the solutions of the two inequalities on the number line before writing the solution of the compound inequality in the interval notation. By having a visual representation of how the two inequalities behave on the number line, it is much easier to write its corresponding interval notation.

We will also go over some examples where the compound inequality has no solution or infinite solution.

Somewhere in our examples, we will discuss an “and” compound inequality case which can be condensed into a single inequality with three parts: left side, middle part, and right side. An example would be - 1 \le x \le 3 which is derived from -1 \le x and x \le 3. By writing it in this form, it can allow us to solve the compound inequality much quicker.

The “AND” Compound Inequalities

Solve the compound “and” inequality by solving each of the two inequalities separately then examine or consider their solutions altogether. For the “and” case, we want to find all the numbers or values that can make both the two inequalities true.

Example 1: Solve the compound inequality x - 1 > 1 and 27 \ge 2x - 1. Graph the solutions on the number line. Then, write your solutions in interval notation.

STEP 1. Solve each inequality.

• First inequality: x - 1 > 1

Add 1 to both sides of the inequality.

x - 1 > 1

x - 1+1 > 1+1

\color{red}x > 2

• Second inequality: 27 \ge 2x - 1

Add both sides of the inequality by 1 then divide by 2. Finally, make sure that the variable is on the left side. When you swap location, in this case, the variable x will move from right to left. The relative orientation of the inequality symbol should remain the same to keep the meaning unchanged. One way to think about it is that the “mouth” of the inequality symbol is opening towards the number 14. So when you swap, the “mouth” of the inequality must still be pointing towards 14.

27 \ge 2x - 1

27 + 1 \ge 2x - 1 + 1

8 \ge 2x

{\Large{{{28} \over 2}}} \ge {\Large{{{2x} \over 2}}}

14 \ge x

\color{red}x \le 14

The solutions are given by \color{red}x > 2 and \color{red}x \le 14.

STEP 2. Graph the solutions on the number line.

For \color{red}x > 2, the point 2 is not included as part of the solutions since x > 2 means all numbers greater than 2. In addition, it does not have any conditions of equality that’s why we must exclude the number 2. So we will put an open circle over 2 to indicate that it is not a solution. The solutions are all numbers greater than 2, thus we draw an arrow to the right of 2.

For \color{red}x \le 14, we read it as “x is less than or equal to 14“. Notice there is a condition of equality, therefore the number 14 is part of the solution so we will put a closed circle over it. All numbers to the left of 14 are also solutions so we will draw an arrow pointing to the left of it.

The final solutions will be the intersection or overlap of the two inequalities: \color{red}x > 2 and \color{red}x \le 14. Notice that all the numbers between 2 and 14 intersect so they are part of the final solutions of the “and” compound inequality. They also intersect at the number 14 so we add it in the solution set. However, they do not intersect at point 2, thus we drop it off as part of the solutions. We have just figured out the complete solution set of the given compound inequality.

STEP 3. Write the solutions in interval notation.

Observe that all numbers between 2 and 14 are part of the solutions. In addition, the number 2 is excluded because it is with an open circle while 14 is included because it is covered with a closed circle. Now, we use a rounded bracket or parenthesis if it is excluded (2 is excluded), and use a square bracket if included (14 is included).

\Large{\left( {2,14} \right]}

It is read as “all the numbers greater than 2 but less than or equal to 14“.

Remember: This type of interval is also known as half-closed or half-open interval because one of the two endpoints is included but the other is not.

Example 2: Solve the compound inequality 2 + 3x > - 10 and 2\left( {x - 1} \right) < x + 4. Graph the solution set on the number line. Then, write the solution set in the interval notation.

STEP 1. Solve each inequality.

• First inequality: 2 + 3x > - 10

Subtract both sides of the inequality by 2. Then divide both sides by 3.

2 + 3x > - 10

2 - 2 + 3x > - 10 - 2

3x > - 12

{\Large{{{3x} \over 3}}} > {\Large{{{ - 12} \over 3}}}

\color{red}x > - \,4

• Second Inequality: 2\left( {x - 1} \right) < x + 4

Distribute the 2 to the binomial inside the parenthesis. Add 2 on both sides by of the inequality. Then subtract both sides by x.

2\left( {x - 1} \right) < x + 4

2x - 2 < x + 4

2x - 2 + 2 < x + 4 + 2

2x < x + 6

2x - x < x - x + 6

\color{red}x < 6

The solutions are given by \color{red}x > - \,4 and \color{red}x < 6.

STEP 2. Graph the solution set on the number line.

A strict inequality is a type of inequality that is either absolutely greater than a number, x>a, or absolutely less than a number, x<a. Notice that strict inequality does not contain any equality component.

On the other hand, the inequality symbol x \ge a which is read as “x is greater than or equal to a and the inequality symbol x \le a which is read as “x is less than or equal to a” are both non-strict inequalities because they have the equality conditions.

The inequality \color{red}x > - \,4 is a strict inequality therefore we will put an open circle over -4 as it is not part of the solutions, and draw an arrow to the right. Similarly, \color{red}x < 6 is a strict inequality thus we will put an open circle over 6 , and draw an arrow to the left.

The final solution set will be the intersection of \color{red}x > - \,4 and \color{red}x < 6 which are all the numbers between -4 and 6 but excluding the endpoints -4 and 6.

STEP 3. Write the solutions in interval notation.

We will use rounded brackets or parentheses on both sides to signify that both endpoints are excluded from the solution set.

\Large{\left( {-4,6} \right)}

It is read as “all the numbers greater than -4 but less than 6“.

Remember: This type of interval is also known as an open interval because the two endpoints are excluded in the solution set. That is, they are NOT part of the solutions.

Example 3: Solve the compound inequality 5 - 3\left( {x - 2} \right) \le x - \left( { - 2x + 13} \right) and 5 - \left( {x + 1} \right) \le 2\left( {7 - x} \right) + 1. Graph the solution set then write its solutions in the interval notation.

STEP 1: Solve each inequality.

• First inequality: 5 - 3\left( {x - 2} \right) \le x - \left( { - 2x + 13} \right)

Get rid of the parenthesis on each side of the inequality using the Distributive Property of Multiplication over Addition. Add 5 and 6 on the left. Subtract both sides by 11. Subtract both sides by 3x. To solve x, divide both sides by -6. Since we divide each side by a negative number, we will switch the direction of the inequality. That is, “from less than or equal to” to “greater than or equal to”.

5 - 3\left( {x - 2} \right) \le x - \left( { - 2x + 13} \right)

5 - 3x + 6 \le x + 2x - 13 

- 3x + 11 \le 3x - 13  

- 3x + 11 - 11 \le 3x - 13 - 11

- 3x \le 3x - 24 

- 3x -3x \le 3x - 3x+24 

- 6x \le-24

{\Large{{{ - 6x} \over { - 6}}}} \le {\Large{{{ - 24} \over { - 6}}}}

\color{red}x \ge 4

• Second Inequality: 5 - \left( {x + 1} \right) \le 2\left( {7 - x} \right) + 1

Eliminate the parentheses using the Distributive Property of Multiplication over Addition. Subtract 5 by 1 on the left side. Subtract 4 to both sides of the inequality. Then add 2x to both sides to finish it off.

5 - \left( {x + 1} \right) \le 2\left( {7 - x} \right) + 1

5 - x - 1 \le 14 - 2x + 1 

4 - x \le 15 - 2x 

4 - 4 - x \le 15 - 4 - 2x 

- x \le 11 - 2x 

- x + 2x \le 11 - 2x + 2x

\color{red}x \le 11

The solutions are given by \color{red}x \ge 4 and \color{red}x \le 11 .

STEP 2. Graph the solution set on the number line.

For \color{red}x \ge 4, we will shade the circle above 4 to show that it is included in the solutions because the inequality has a condition of equality, that is, “greater than or equal to”. The arrow points to the right of 4 because it has a component of “greater than”.

For \color{red}x \le 11 , we will also shade the circle above 11 to indicate that is it is part of the solution set since the inequality has a condition of equality, that is, “less than or equal to”. The arrow points to the left of 11 because it is of the case of less than.

As for the final solution set, we find all the points where the two inequalities intersect. Obviously, they intersect between 4 and 11. More so, they also overlap at the endpoints. Therefore, the final solution set contains all the points between the endpoints 4 and 11 and including the endpoints.

STEP 3. Write the solution set in interval notation.

We will use square brackets on both sides to signify that both endpoints are included in the solution set.

\Large{\left[ {4,11} \right]}

It is read as “all numbers greater than or equal to 4 but less than or equal to 11“.

Remember: This type of interval is also known as a closed interval because the two endpoints are included in the solution set. That is, they are part of the solutions.

Example 4: Solve the compound inequality 3x - 2\left( {1 - x} \right) < x - 6 and 10 - x < x + 2. Graph the solution set then write its solutions in the interval notation.

STEP 1. Solve each inequality.

• First inequality: 3x - 2\left( {1 - x} \right) < x - 6

Distribute the -2 into the binomial 1-x on the left side of the inequality to remove the parenthesis. Add 3x and 2x on the left side as well. Add 2 to both sides of the inequality. Subtract x on both sides. Finally, divide both sides of the inequality by 4.

3x - 2\left( {1 - x} \right) < x - 6

3x - 2 + 2x < x - 6

5x - 2 < x - 6

5x - 2 + 2 < x - 6 + 2

5x < x - 4

5x - x < x - x - 4

4x <  - 4

{\Large{{{4x} \over 4}}} < {\Large{{{ - 4} \over 4}}}

\color{red}x <- 1

• Second inequality: 10 - x < x + 2

Subtract both sides of the inequality by 10. Then, subtract it also on both sides by x. Finally, divide each side by -2. Since we are dividing by a negative number, we must flip or switch the direction of the inequality symbol. In this case, from less than to greater than.

10 - x < x + 2

10 - 10 - x < x + 2 - 10

- x < x - 8

- x - x < x - x - 8

- 2x <  - 8

{\Large{{{ - 2x} \over { - 2}}}} > {\Large{{{ - 8} \over { - 2}}}}

\color{red} x > 4

Observe that the solutions of the two inequalities \color{red}x <- 1 and \color{red} x > 4 do not intersect, and thus the compound inequality has no solution.

It is more obvious that they don’t overlap if we look at their graphs on the number line.

Example 5: Solve the compound inequality - 5 < 3x + 7 \le 22. Graph the solution set then write its solutions in the interval notation.

This hybrid-looking inequality which is comprised of two inequality symbols and three parts is actually a combination of two inequalities joined together by an “AND” conjunction.

We can separate this compound inequality into two inequalities with an “AND” connector and then solve them as usual. This is how it looks if we break the compound inequality into two simpler inequalities.

However, there is no need to separate it into two inequalities. We can solve the compound inequality as it stands. In fact, I like it the way it is because it is much easier to solve.

The goal is to isolate the variable of the middle part. To keep everything balanced, whatever we do on the middle part, we must do the same on the left side and the right side. When it is done properly, the answer should come out nicely!

Here we go. Let’s work it out!

STEP 1: Solve the compound inequality.

To solve for x, we subtract the middle by 7 which means we have to do the same with the left and right side of the compound inequality. Finally, to isolate x, we divide the middle by 3 which we will do the same to the left and right.

- 5 < 3x + 7 \le 22

- 5 {\color{red}- 7} < 3x + 7{\color{red} - 7} \le 22{\color{red} - 7} 

- 12 < 3x \le 15

{\Large{{{ - 15} \over {\color{red}3}}}} < {\Large{{{3x} \over {\color{red}3}}}} \le {\Large{{{15} \over {\color{red}3}}}}

- 4 < x \le 5

The solutions are given by - 4 < x \le 5.

STEP 2. Graph the solution set on the number line.

STEP 3. Write the solutions in interval notation.

\Large{\left( {-4,5} \right]}

It is read as “all numbers greater than -4 but less than or equal to 5“.

The “OR” Compound Inequalities

Solve the compound “or” inequality by solving each of the two inequalities separately. For the “or” case, we want to find all the numbers that can make at least one of the two inequalities to be true.

How do you write a solution to a compound inequality?

What is an example of a compound inequality?

What is a compound inequality?