Identifying Rational and Irrational Numbers
Learning Outcomes
- Identify rational numbers from a list of numbers
- Identify irrational numbers from a list of numbers
| counting numbers | [latex]1,2,3,4\dots [/latex] |
| whole numbers | [latex]0,1,2,3,4\dots[/latex] |
| integers | [latex]\dots -3,-2,-1,0,1,2,3,4\dots [/latex] |
Rational Numbers
What type of numbers would you get if you started with all the integers and then included all the fractions? The numbers you would have form the set of rational numbers. A rational number is a number that can be written as a ratio of two integers.Rational Numbers
A rational number is a number that can be written in the form [latex]{\Large\frac{p}{q}}[/latex], where [latex]p[/latex] and [latex]q[/latex] are integers and [latex]q\ne o[/latex].[latex]\Large\frac{4}{5}\normalsize ,-\Large\frac{7}{8}\normalsize ,\Large\frac{13}{4}\normalsize ,\text{and}-\Large\frac{20}{3}[/latex]
Each numerator and each denominator is an integer. We need to look at all the numbers we have used so far and verify that they are rational. The definition of rational numbers tells us that all fractions are rational. We will now look at the counting numbers, whole numbers, integers, and decimals to make sure they are rational. Are integers rational numbers? To decide if an integer is a rational number, we try to write it as a ratio of two integers. An easy way to do this is to write it as a fraction with denominator one.[latex]3=\Large\frac{3}{1}\normalsize ,\space-8=\Large\frac{-8}{1}\normalsize ,\space0=\Large\frac{0}{1}[/latex]
Since any integer can be written as the ratio of two integers, all integers are rational numbers. Remember that all the counting numbers and all the whole numbers are also integers, and so they, too, are rational. What about decimals? Are they rational? Let's look at a few to see if we can write each of them as the ratio of two integers. We've already seen that integers are rational numbers. The integer [latex]-8[/latex] could be written as the decimal [latex]-8.0[/latex]. So, clearly, some decimals are rational. Think about the decimal [latex]7.3[/latex]. Can we write it as a ratio of two integers? Because [latex]7.3[/latex] means [latex]7\Large\frac{3}{10}[/latex], we can write it as an improper fraction, [latex]\Large\frac{73}{10}[/latex]. So [latex]7.3[/latex] is the ratio of the integers [latex]73[/latex] and [latex]10[/latex]. It is a rational number. In general, any decimal that ends after a number of digits such as [latex]7.3[/latex] or [latex]-1.2684[/latex] is a rational number. We can use the place value of the last digit as the denominator when writing the decimal as a fraction.example
Write each as the ratio of two integers: 1. [latex]-15[/latex] 2. [latex]6.81[/latex] 3. [latex]-3\Large\frac{6}{7}[/latex] Solution:| 1. | |
| [latex]-15[/latex] | |
| Write the integer as a fraction with denominator 1. | [latex]\Large\frac{-15}{1}[/latex] |
| 2. | |
| [latex]6.81[/latex] | |
| Write the decimal as a mixed number. | [latex]6\Large\frac{81}{100}[/latex] |
| Then convert it to an improper fraction. | [latex]\Large\frac{681}{100}[/latex] |
| 3. | |
| [latex]-3\Large\frac{6}{7}[/latex] | |
| Convert the mixed number to an improper fraction. | [latex]-\Large\frac{27}{7}[/latex] |
try it
[ohm_question]145911[/ohm_question]Integer [latex]-2,-1,0,1,2,3[/latex]
Decimal [latex]-2.0,-1.0,0.0,1.0,2.0,3.0[/latex]
These decimal numbers stop. We have also seen that every fraction is a rational number. Look at the decimal form of the fractions we just considered.Ratio of Integers [latex]\Large\frac{4}{5}\normalsize ,\Large\frac{7}{8}\normalsize ,\Large\frac{13}{4}\normalsize ,\Large\frac{20}{3}[/latex]
Decimal Forms [latex]0.8,-0.875,3.25,-6.666\ldots,-6.\overline{66}[/latex]
These decimals either stop or repeat. What do these examples tell you? Every rational number can be written both as a ratio of integers and as a decimal that either stops or repeats. The table below shows the numbers we looked at expressed as a ratio of integers and as a decimal.| Rational Numbers | ||
|---|---|---|
| Fractions | Integers | |
| Number | [latex]\Large\frac{4}{5}\normalsize ,-\Large\frac{7}{8}\normalsize ,\Large\frac{13}{4}\normalsize ,\Large\frac{-20}{3}[/latex] | [latex]-2,-1,0,1,2,3[/latex] |
| Ratio of Integer | [latex]\Large\frac{4}{5}\normalsize ,\Large\frac{-7}{8}\normalsize ,\Large\frac{13}{4}\normalsize ,\Large\frac{-20}{3}[/latex] | [latex]\Large\frac{-2}{1}\normalsize ,\Large\frac{-1}{1}\normalsize ,\Large\frac{0}{1}\normalsize ,\Large\frac{1}{1}\normalsize ,\Large\frac{2}{1}\normalsize ,\Large\frac{3}{1}[/latex] |
| Decimal number | [latex]0.8,-0.875,3.25,-6.\overline{6}[/latex] | [latex]-2.0,-1.0,0.0,1.0,2.0,3.0[/latex] |
Irrational Numbers
Are there any decimals that do not stop or repeat? Yes. The number [latex]\pi [/latex] (the Greek letter pi, pronounced ‘pie’), which is very important in describing circles, has a decimal form that does not stop or repeat.[latex]\pi =\text{3.141592654.......}[/latex]
Similarly, the decimal representations of square roots of numbers that are not perfect squares never stop and never repeat. For example,[latex]\sqrt{5}=\text{2.236067978.....}[/latex]
A decimal that does not stop and does not repeat cannot be written as the ratio of integers. We call this kind of number an irrational number.Irrational Number
An irrational number is a number that cannot be written as the ratio of two integers. Its decimal form does not stop and does not repeat.- stops or repeats, the number is rational.
- does not stop and does not repeat, the number is irrational.
example
Identify each of the following as rational or irrational: 1. [latex]0.58\overline{3}[/latex] 2. [latex]0.475[/latex] 3. [latex]3.605551275\dots [/latex]Answer: Solution: 1. [latex]0.58\overline{3}[/latex] The bar above the [latex]3[/latex] indicates that it repeats. Therefore, [latex]0.58\overline{3}[/latex] is a repeating decimal, and is therefore a rational number. 2. [latex]0.475[/latex] This decimal stops after the [latex]5[/latex], so it is a rational number. 3. [latex]3.605551275\dots[/latex] The ellipsis [latex](\dots)[/latex] means that this number does not stop. There is no repeating pattern of digits. Since the number doesn't stop and doesn't repeat, it is irrational.
try it
[ohm_question]145910[/ohm_question]example
Identify each of the following as rational or irrational: 1. [latex]\sqrt{36}[/latex] 2. [latex]\sqrt{44}[/latex]Answer: Solution: 1. The number [latex]36[/latex] is a perfect square, since [latex]{6}^{2}=36[/latex]. So [latex]\sqrt{36}=6[/latex]. Therefore [latex]\sqrt{36}[/latex] is rational. 2. Remember that [latex]{6}^{2}=36[/latex] and [latex]{7}^{2}=49[/latex], so [latex]44[/latex] is not a perfect square. This means [latex]\sqrt{44}[/latex] is irrational.
try it
[ohm_question]145915[/ohm_question]Licenses & Attributions
CC licensed content, Original
- Determine Rational or Irrational Numbers (Square Roots and Decimals Only). Authored by: James Sousa (Mathispower4u.com) for Lumen Learning. License: CC BY: Attribution.
- Question ID 145910, 145915, 145911. Authored by: Lumen Learning. License: CC BY: Attribution.
CC licensed content, Shared previously
- Real Numbers. Authored by: James Sousa (Mathispower4u.com). License: CC BY: Attribution.
CC licensed content, Specific attribution
- Prealgebra. Provided by: OpenStax License: CC BY: Attribution. License terms: Download for free at http://cnx.org/contents/[email protected].
