Compound inequalities are inequalities bounded by more than one condition. What does it mean? Simple inequalities have one unknown, certain value and an inequality sign between them. In compound inequalities there are two inequality signs. This may seem complicated, but there is no reason to worry. We can simply break down one inequality into two simple ones which know to solve. But, we need to be careful…
In this lesson, there are basically only two different type of problems: one in which the conditions are bounded with an operation ‘AND’ and second in which the conditions are bounded with an ‘OR’.
Inequalities whose conditions are bounded with “and” are not independent of each other. That means the final solution will be the intersection of solutions of separate inequalities. When you will be working with inequalities you will always get a set of numbers as a solution.
They will usually come in the following form: condition1 <(≤) x <(≤) contition2, which can be w broken down into two simple inequalities: condition1 <(≤) x AND x <(≤) contition2.
Which means that now you have: and . Solve them separately, write down their solutions and draw them on the number line.
1. => =>
For every compound inequality in this form the solution will be the common solution of the inequalities you got by dividing it. Always draw your solutions on the number line; that is the best way for you to see the solution. The solution will be the part of a number line where the lines of solutions overlap.
In this example our solution will be . Also don’t forget the difference between different brackets and if you are considering the endpoints as parts of the solution or not.
Now take a look at a bit more complicated example 2:
First is the same way you did the last example. Divide into two inequalities and solve separately, the solution will again be the intersection between the sets of solutions of those two inequalities.
If we draw those solutions on a number line we get:
So our solution is the set .
Now, if you are already comfortable with inequalities you can do the following and solve it a little bit faster. If you have a inequality like this, and want to do some operations on it, you have to do it for every part, for example if you’re going to add something on the left, you have to add it in the middle and on the right. If you noticed at first that you have an always true statement you can subtract from every part -2x and then you’ll have
is again always true statement, so we only consider .
You can also use this procedure to simplify, if you can, your inequalities. For example if you have something like example 3.
If you subtract from every part you get .
And then you simply continue with the procedure you learned before.
and (here we came across a minus, if you are multiplying or dividing with minus the inequality sign must change)
The final solution is the set
In case your lines don’t overlap your inequality will have no solutions.
For example if you get and .
Also, you can get different forms of inequalities.
You solve this in the same way. and → .
There are also compound inequalities whose conditions are bounded with an ‘or’. The only difference between solving them and solving those with an ‘and’ is that in your final solution you will look for a union of separate inequalities.
The final solution is the union between these two sets: .
In cases like this, you don’t have to write to separate sets because one contains another. The solution will be the greater one in this case the solution is .
Compound inequalities worksheets
Integers (946.4 KiB, 217 hits)
Decimals (1.0 MiB, 215 hits)
Fractions (1.2 MiB, 204 hits)