### Learning Outcomes

- Use interval notation to describe intersections and unions
- Use graphs to describe intersections and unions

## Use Interval Notation to Describe Sets of Numbers as Intersections and Unions

When two inequalities are joined by the word *and*, the solution of the compound inequality occurs when *both* inequalities are true at the same time. It is the overlap, or intersection, of the solutions for each inequality. When the two inequalities are joined by the word *or*, the solution of the compound inequality occurs when *either* of the inequalities is true. The solution is the combination, or union, of the two individual solutions.

In this section, we will learn how to solve compound inequalities that are joined with the words AND and OR. First, it will help to see some examples of inequalities, intervals, and graphs of compound inequalities. This will help you describe the solutions to compound inequalities properly.

Venn diagrams use the concept of intersections and unions to compare two or more things. For example, this Venn diagram shows the intersection of people who are breaking your heart and those who are shaking your confidence daily. Apparently, Cecilia has both of these qualities; therefore, she is the intersection of the two.

In mathematical terms, consider the inequality [latex]x\lt6[/latex] and [latex]x\gt2[/latex]. How would we interpret what numbers [latex]x[/latex] can be, and what would the interval look like?

In words, [latex]x[/latex] must be less than [latex]6[/latex], and at the same time, it must be greater than [latex]2[/latex]. This is much like the Venn diagram above, where Cecilia is at once breaking your heart and shaking your confidence daily. Now look at a graph to see what numbers are possible with these constraints.

The numbers that are shared by both lines on the graph are called the intersection of the two inequalities [latex]x\lt6[/latex] and [latex]x\gt2[/latex]. This is called a *bounded* inequality, and is written as [latex]2\lt{x}\lt6[/latex]. Think about that one for a minute. [latex]x[/latex] must be less than [latex]6[/latex] and greater than two—the values for [latex]x[/latex] will fall *between two numbers.* In interval notation, this looks like [latex]\left(2,6\right)[/latex]. The graph would look like this:

In mathematical terms, for example, [latex]x>6[/latex] *or* [latex]x<2[/latex] is an inequality joined by the word *or*. Using interval notation, we can describe each of these inequalities separately:

[latex]x\gt6[/latex] is the same as [latex]\left(6, \infty\right)[/latex] and [latex]x<2[/latex] is the same as [latex]\left(-\infty, 2\right)[/latex]. If we are describing solutions to inequalities, what effect does the *or *have? We are saying that solutions are either real numbers less than two *or* real numbers greater than [latex]6[/latex]. Can you see why we need to write them as two separate intervals? Let us look at a graph to get a clear picture of what is going on.

When you place both of these inequalities on a graph, we can see that they share no numbers in common. As mentioned above, this is what we call a union. The interval notation associated with a union is a big U, so instead of writing *or*, we join our intervals with a big U, like this:

[latex]\left(-\infty, 2\right)\cup\left(6, \infty\right)[/latex]

It is common convention to construct intervals starting with the value that is furthest left on the number line as the left value, such as [latex]\left(2,6\right)[/latex], where [latex]2[/latex] is less than [latex]6[/latex]. The number on the right should be greater than the number on the left.

### Try It

### Example

Draw the graph of the compound inequality [latex]x\gt3[/latex] *or* [latex]x\le4[/latex] and describe the set of *x*-values that will satisfy it with an interval.

In the following video, you will see two examples of how to express inequalities involving *or* graphically and as an interval.

### Examples

Draw a graph of the compound inequality: [latex]x\lt5[/latex] *and* [latex]x\ge−1[/latex], and describe the set of *x*-values that will satisfy it with an interval.

### Examples

Draw the graph of the compound inequality [latex]x\lt{-3}[/latex] *and* [latex]x\gt{3}[/latex], and describe the set of *x*-values that will satisfy it with an interval.

The following video presents two examples of how to draw inequalities involving *and*, as well as write the corresponding intervals.