Several inequalities can be combined together to form what are called compound inequalities.
The first type of compound inequality is the “or” inequality, which is true when either inequality results in a true statement. When graphing this type of inequality, one useful trick is to graph each individual inequality above the number line before moving them both down together onto the actual number line.
When giving interval notation for a solution, if there are two different parts to the graph, put a ∪ (union) symbol between two sets of interval notation, one for each part.
Solve the inequality
Isolate the variables from the numbers:
Isolate the variable
Solution:
Position the inequalities and graph:
In interval notation, the solution is written as
Note: there are several possible results that result from an “or” statement. The graphs could be pointing different directions, as in the graph above, or pointing in the same direction, as in the graph representing
In interval notation, this solution is written as
It is also possible to have solutions that point in opposite directions but are overlapping, as shown by the solutions and graph below.
In interval notation, this solution is written as
The second type of compound inequality is the “and” inequality. “And” inequalities require both inequality statements to be true. If one part is false, the whole inequality is false. When graphing these inequalities, follow a similar process as before, sketching both solutions for both inequalities above the number line. However, this time, it is only the overlapping portion that is drawn onto the number line. When a solution for an “and” compound inequality is given in interval notation, it will be expressed in a manner very similar to single inequalities. The symbol that can be used for “and” is the intersection symbol, ∩.
Solve the compound inequality
Move all variables to the right side and all numbers to the left:
Isolate the variable
Solution:
In interval notation, this solution is written as
Note: there are several different results that could result from an “and” statement. The graphs could be pointing towards each other as in the graph above, or pointing in the same direction, as in the graph representing
In interval notation, this solution is written as
It is also possible to have solutions that point in opposite directions but do not overlap, as shown by the solutions and graph below. Since there is no overlap, there is no real solution.
In interval notation, this solution is written as no solution,
The third type of compound inequality is a special type of “and” inequality. When the variable (or expression containing the variable) is between two numbers, write it as a single math sentence with three parts, such as
Solve the inequality
Isolate the variable
Isolate the variable
In interval notation, this is written as
Questions
For questions 1 to 32, solve each compound inequality, graph its solution, and write it in interval notation.
Answer Key 4.2