Mathematics » Properties of Real Numbers » Rational and Irrational Numbers

Identifying Rational Numbers and Irrational Numbers

Identifying Rational Numbers and Irrational Numbers

Congratulations! You have completed the first six tutorials of this book! It’s time to take stock of what you have done so far in this course and think about what is ahead. You have learned how to add, subtract, multiply, and divide whole numbers, fractions, integers, and decimals. You have become familiar with the language and symbols of algebra, and have simplified and evaluated algebraic expressions. You have solved many different types of applications. You have established a good solid foundation that you need so you can be successful in algebra.

In this tutorial, we’ll make sure your skills are firmly set. We’ll take another look at the kinds of numbers we have worked with in all previous tutorials. We’ll work with properties of numbers that will help you improve your number sense. And we’ll practice using them in ways that we’ll use when we solve equations and complete other procedures in algebra.

We have already described numbers as counting numbers, whole numbers, and integers. Do you remember what the difference is among these types of numbers?

counting numbers\(1,2,3,\text{4…}\)
whole numbers\(0,1,2,3,\text{4…}\)
integers\(\text{…}-3,-2,-1,0,1,2,3,\text{4…}\)

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.

Definition: Rational Numbers

A rational number is a number that can be written in the form \(\frac{p}{q},\) where \(p\) and \(q\) are integers and \(q\ne o.\)

All fractions, both positive and negative, are rational numbers. A few examples are

\(\frac{4}{5},-\frac{7}{8},\frac{13}{4},\text{and}\phantom{\rule{0.2em}{0ex}}-\frac{20}{3}\)

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.

\(3=\frac{3}{1}\phantom{\rule{2em}{0ex}}-8=\frac{-8}{1}\phantom{\rule{2em}{0ex}}0=\frac{0}{1}\)

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 \(-8\) could be written as the decimal \(-8.0.\) So, clearly, some decimals are rational.

Think about the decimal \(7.3.\) Can we write it as a ratio of two integers? Because \(7.3\) means \(7\frac{3}{10},\) we can write it as an improper fraction, \(\frac{73}{10}.\) So \(7.3\) is the ratio of the integers \(73\) and \(10.\) It is a rational number.

In general, any decimal that ends after a number of digits (such as \(7.3\) or \(-1.2684\)) 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. \(-15\)
  2. \(\phantom{\rule{0.2em}{0ex}}6.81\)
  3. \(\phantom{\rule{0.2em}{0ex}}-3\frac{6}{7}.\)

Solution

(a) 
 \(-15\)
Write the integer as a fraction with denominator 1.\(\frac{-15}{1}\)
(b) 
 \(6.81\)
Write the decimal as a mixed number.\(6\frac{81}{100}\)
Then convert it to an improper fraction.\(\frac{681}{100}\)
(c) 
 \(-3\frac{6}{7}\)
Convert the mixed number to an improper fraction.\(-\frac{27}{7}\)

Let’s look at the decimal form of the numbers we know are rational. We have seen that every integer is a rational number, since \(a=\frac{a}{1}\) for any integer, \(a.\) We can also change any integer to a decimal by adding a decimal point and a zero.

\(\begin{array}{cccc}\text{Integer}\hfill & & & -2,-1,0,1,2,3\hfill \\ \text{Decimal}\hfill & & & -2.0,-1.0,0.0,1.0,2.0,3.0\phantom{\rule{1.0em}{0ex}}\text{These decimal numbers stop.}\hfill \end{array}\)

We have also seen that every fraction is a rational number. Look at the decimal form of the fractions we just considered.

\(\begin{array}{cccc}\text{Ratio of Integers}\hfill & & & \frac{4}{5}, & -\frac{7}{8}, & \frac{13}{4}, & -\frac{20}{3}\hfill \\ \text{Decimal Forms}\hfill & & & 0.8, & -0.875, & 3.25, & -6.666… \phantom{\rule{1.0em}{0ex}}\text{These decimals either stop or repeat.}\hfill \end{array}\)

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
 FractionsIntegers
Number\(\frac{4}{5},-\frac{7}{8},\frac{13}{4},\frac{-20}{3}\)\(-2,-1,0,1,2,3\)
Ratio of Integer\(\frac{4}{5},\frac{-7}{8},\frac{13}{4},\frac{-20}{3}\)\(\frac{-2}{1},\frac{-1}{1},\frac{0}{1},\frac{1}{1},\frac{2}{1},\frac{3}{1}\)
Decimal number\(0.8,-0.875,3.25,-6.\stackrel{\text{–}}{\text{6}},\)\(-2.0,-1.0,0.0,1.0,2.0,3.0\)

Irrational Numbers

Are there any decimals that do not stop or repeat? Yes. The number \(\pi \) (the Greek letter pi, pronounced ‘pie’), which is very important in describing circles, has a decimal form that does not stop or repeat.

\(\pi =\text{3.141592654…….}\)

Similarly, the decimal representations of square roots of numbers that are not perfect squares never stop and never repeat. For example,

\(\sqrt{5}=\text{2.236067978…..}\)

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.

Definition: 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.

Let’s summarize a method we can use to determine whether a number is rational or irrational.

If the decimal form of a number

  • 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. \(\phantom{\rule{0.2em}{0ex}}0.58\stackrel{\text{–}}{\text{3}}\)
  2. \(\phantom{\rule{0.2em}{0ex}}0.475\)
  3. \(\phantom{\rule{0.2em}{0ex}}\text{3.605551275…}\)

Solution

\(0.58\stackrel{\text{–}}{\text{3}}\)

The bar above the \(3\) indicates that it repeats. Therefore, \(0.58\stackrel{\text{–}}{\text{3}}\) is a repeating decimal, and is therefore a rational number.

\(0.475\)

This decimal stops after the \(5\), so it is a rational number.

\(\text{3.605551275…}\)

The ellipsis \(\text{(…)}\) 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.

Let’s think about square roots now. Square roots of perfect squares are always whole numbers, so they are rational. But the decimal forms of square roots of numbers that are not perfect squares never stop and never repeat, so these square roots are irrational.

Example

Identify each of the following as rational or irrational:

  1. \(\phantom{\rule{0.2em}{0ex}}\sqrt{36}\)
  2. \(\phantom{\rule{0.2em}{0ex}}\sqrt{44}\)

Solution

The number \(36\) is a perfect square, since \({6}^{2}=36.\) So \(\sqrt{36}=6.\) Therefore \(\sqrt{36}\) is rational.

Remember that \({6}^{2}=36\) and \({7}^{2}=49,\) so \(44\) is not a perfect square.

This means \(\sqrt{44}\) is irrational.

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