The average weight gain of an infant, after 6 months of age, is one pound a month, until the age of 2. If the average 6-month-old weighs 16 pounds, up to what age would an infant weigh 25 pounds or less?
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Solving a linear inequality is very similar to solving a linear equality, or equation. There are a few very important differences. We no longer use an equal sign. There are four different inequality signs, shown below.
Less than or equal to
Greater than or equal to
Notice that the line underneath the and signs indicates “equal to.” The inequality would be read, “ is greater than -1.” We can also graph these solutions on a number line. To graph an inequality on a number line, shading is used. This is because an inequality is a range of solutions, not just one specific number. To graph , it would look like this:
Notice that the circle at -1 is open. This is to indicate that -1 is not included in the solution. A < sign would also have an open circle. If the inequality was a or sign, then the circle would be closed, or filled in. Shading to the right of the circle shows that any number greater than -1 will be a solution to this inequality.
Is a solution to ?
Solution: Plug in -8 for and test this solution.
Of course, 2 cannot be greater than 3. Therefore, this is not a valid solution.
Solve and graph the solution to .
Solution: For the most part, solving an inequality is the same as solving an equation. The major difference will be addressed in Example C. This inequality can be solved just like an equation.
Test a solution,
Plotting the solution, we get:
Always test a solution that is in the solution range. It will help you determine if you solved the problem correctly.
Solve and graph .
Solution: When solving inequalities, be careful with negatives. Let’s solve this problem the way we normally would an equation.
Let’s check a solution. If is less than or equal to 6, let’s test 1.
This is not a true inequality. To make this true, we must flip the inequality. Therefore, whenever we multiply or divide by a negative number, we must flip the inequality sign. The answer to this inequality is actually . Now, let’s test a number in this range.
This is true. The graph of the solution is:
Intro Problem Revisit First, write an inequality. Let m represent the age of the infant, in months. Remember, when you get the final answer, you must add 6 for the initial weight of the infant at 6 months.
Adding 6, we have . So, up to 15 months, the average baby should weigh 25 pounds or less.
1. Is a solution to ?
Solve the following inequalities and graph.
1. Plug -5 into the inequality.
This is true because 22 is larger than 12. -5 is a solution.
2. No negatives with the term, so we can solve this inequality like an equation.
Test a solution,
The graph looks like:
3. In this inequality, we have a negative term. Therefore, we will need to flip the inequality.
Test a solution,
Notice that we FLIPPED the inequality sign when we DIVIDED by -1. Also, this equation can also be written . Here is the graph:
Solve and graph each inequality.
For questions 16 and 17, write the inequality statement given by the graph below.
Answers for Explore More Problems
To view the Explore More answers, open this PDF file and look for section 1.10.