4.14: Properties of Rational Numbers
Have you ever compared numbers?
Molly has spent the day skiing. She did 12 runs on the diamond trails and was very pleased with her speed and ability. She felt that out of the 12 runs, that 9 of them were particularly good.
She wrote \begin{align*}\frac{9}{12}\end{align*}.
Besides being a fraction, this is another type of number too.
Do you know what it is?
This Concept will help you to understand rational numbers.
Guidance
Some numbers are considered rational numbers. A rational number is a number that can be written as a ratio.
What is a ratio?
A ratio is a comparison of two numbers. For example, you might discover that the ratio of boys to girls in your class one day was 12 to 13. That same ratio could be also be expressed using a colon, 12 : 13, or as a fraction, \begin{align*}\frac{12}{13}\end{align*}.
In fact, any number that can be written as the ratio of two integers is classified as a rational number. Let's take a closer look at how to identify rational numbers now.
How can we determine if an integer is a rational number?
That is a good question. Let’s look at a value and see if we can write it as a ratio.
10
This number can be written as a ratio. Each whole number can be written over 1. That means that it can be written in the form of a ratio. Notice that the fraction bar is a way to tell if the integer can be written as a ratio.
10 is a rational number.
\begin{align*}-\frac{2}{3}\end{align*}
This fraction is a rational number. Notice that it is written as a ratio already. We are comparing the numerator and the denominator. Yes, it is negative. But that is okay, because we can have negative fractions. We call them rational numbers.
\begin{align*}-\frac{2}{3}\end{align*} is a rational number.
What about a decimal?
.687
This decimal can be written as a rational number over 1000. This is a rational number too.
.687 is a rational number.
Are there any others?
Yes. Terminating decimals and repeating decimals are also rational numbers.
- Terminating decimals, which are decimals with a set number of digits, are always rational. For example, 0.007 is a terminating decimal, so it is rational.
- Repeating decimals, which are decimals in which one or more digits repeat, are always rational. For example, \begin{align*}0.\bar{3}\end{align*} is a repeating decimal in which the digit 3 repeats forever, so it is rational.
Are there any numbers that are not rational?
Yes. Some decimals don’t terminate and they don’t repeat. They just go on and on and on forever. These are a special group of numbers called irrational numbers. They are not rational numbers. You will learn more about them in another Concept.
Determine whether each is a rational number.
Example A
\begin{align*}-4\end{align*}
Solution: Yes
Example B
\begin{align*}\frac{1}{3}\end{align*}
Solution: Yes
Example C
\begin{align*}.89765....\end{align*}
Solution: No, it does not terminate or repeat.
Here is the original problem once again.
Molly has spent the day skiing. She did 12 runs on the diamond trails and was very pleased with her speed and ability. She felt that out of the 12 runs, that 9 of them were particularly good.
She wrote \begin{align*}\frac{9}{12}\end{align*}.
Besides being a fraction, this is another type of number too.
Do you know what it is?
Molly's comparison is a rational number. It is a ratio that is written in fraction form.
Other types of rational numbers are negative numbers, fractions, terminating and repeating decimals.
Vocabulary
Here are the vocabulary words used in this Concept.
- Rational Number
- any number positive or negative that can be written as a ratio.
- Ratio
- a comparison between two quantities. Can be written using the word “to”, using a colon, or using a fraction bar
- Terminating Decimal
- a decimal that has a definite ending
- Repeating Decimal
- a decimal where some of the digits repeat themselves.
- Irrational Number
- a decimal that does not terminate or repeat but continues indefinitely.
Guided Practice
Here is one for you to try on your own.
Show that the following number is rational by writing it as a ratio in fraction form.
\begin{align*}.85\end{align*}
Answer
We can say that this is eighty -five hundredths.
Next, we convert it to a fraction.
\begin{align*}\frac{85}{100}\end{align*}
This is a rational number.
Video Review
Here is a video for review.
- This is a James Sousa video on identifying sets of real numbers including rational numbers.
Practice
Directions: Rewrite each number as the ratio of two integers to prove that each number is rational.
1.\begin{align*}-11\end{align*}
2. \begin{align*}3 \frac{1}{6}\end{align*}
3. \begin{align*}9\end{align*}
4. \begin{align*}.08\end{align*}
5. \begin{align*}-.34\end{align*}
6.\begin{align*}.678\end{align*}
7. \begin{align*}\frac{4}{5}\end{align*}
8.\begin{align*}-19\end{align*}
9. \begin{align*}25\end{align*}
10. \begin{align*}.17\end{align*}
11. \begin{align*}.2347\end{align*}
12. \begin{align*}-17\end{align*}
13. \begin{align*}347\end{align*}
14. \begin{align*}87\end{align*}
15. \begin{align*}-97\end{align*}
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Image Attributions
Here you'll learn to identify a rational number as the ratio of two integers.