Efficiently Solving Inequalities

Student Summary

Here is an inequality: 3(102x)<183(10-2x) < 18. The solution set for this inequality is all the values that can be used in place of xx to make the inequality true. Each solution is one value that makes the inequality true.

In order to solve this inequality, we can first solve the related equation 3(102x)=183(10-2x) = 18 to get the solution x=2x = 2. That means 2 is the boundary between values of xx that make the inequality true and values that make the inequality false.

To solve the inequality, we can check numbers greater than 2 and less than 2 and see which ones make the inequality true.

Let’s check a number that is greater than 2: x=5x= 5. Replacing xx with 5 in the inequality, we get 3(1025)<183(10-2 \boldcdot 5) < 18 or just 0<180 < 18. This is true, so x=5x=5 is a solution. This means that all values greater than 2 make the inequality true. We can represent the solutions as x>2x > 2 and also represent the solutions on a number line:

Number line, labeled negative 3 to 5 by ones.  An arrow begins at 2, open circle, extends to the right.

Notice that 2 itself is not a solution because it's the value of xx that makes 3(102x)3(10-2x) ​equal to 18, and so it does not make 3(102x)<183(10-2x) < 18 true.

For confirmation that we found the correct solution, we can also test a value that is less than 2. If we test x=0x=0, we get 3(1020)<183(10-2 \boldcdot 0) < 18 or just 30<1830 < 18. This is false, so x=0x = 0 and all values of xx that are less than 2 are not solutions.

Visual / Anchor Chart