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6. Terminating and Repeating Decimals
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Chapter 3
6. 

Terminating and Repeating Decimals

This lesson provides a deep dive into the world of decimals, specifically focusing on terminating and repeating decimals. It explains how these types of decimals can be understood through the use of long division and their relation to rational numbers. For example, if one is trying to convert a fraction to a decimal, understanding whether the decimal will terminate or repeat can be crucial. This knowledge is not just theoretical; it has practical applications in various fields such as finance for calculating interest rates, in engineering for precise measurements, and even in everyday scenarios like splitting a bill. The aim is to equip the reader with the tools needed to tackle real-world problems that involve decimals.

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Student Learning Objectives:
  • Identify repeating and terminating decimals
  • Convert between repeating decimals and fractions
12 Theory slides
10 Exercises - Grade E - A
Each lesson is meant to take 1-2 classroom sessions
Terminating and Repeating Decimals
Slide of 12
A rational number can be written as a fraction, mixed number, or decimal number.

ccc Fraction & Mixed Number & Decimal 14/5 & 2 45 & 2.8 Decimal numbers can be grouped by what digits come after the decimal point. For example, what happens when we write 13 or 17 as a decimal? Do any digits repeat? This lesson explores different types of decimal number types and how to write them as fractions.

Catch-Up and Review

Here are a few recommended readings before getting started with this lesson.

Explore

Decimal Forms of Fractions

A rational number can be written as a decimal number using long division. Rewrite the fractions in the table as decimals.

Fraction Decimal
2/9
1/3
3/4
4/11
3/8

Consider the following questions as a guide to classify decimals.

  • Which fractions have a remainder of zero?
  • Which fractions do not have a remainder of zero?
  • Which fractions have the same operations repeated in the long division after a certain point?

Extra

Applet for Calculating the Decimal Numbers
The applet below will help convert the fractions to decimal numbers. To see patterns in the decimal, calculate at least four decimal places.

Applet to compute the division of two numbers

Discussion

Repeating Decimals

A repeating decimal number, or recurring decimal number, is a number in decimal form in which some digits after the decimal point repeat infinitely. The digits repeat their values at regular intervals and the infinitely repeated part is not zero. When writing the decimal, a line is drawn over the repeating portion to indicate the repetition.

Repeating Decimal Numbers
Number Notation Fraction
0.666666... 0.6 2/3
1.533333... 1.53 23/15
5.373737... 5.37 532/99
Since they can be expressed as fractions, repeating decimal numbers are rational numbers.
Discussion

Terminating Decimals

A terminating decimal number is a number in decimal form with a finite number of digits. Terminating numbers can be written as fractions, which means that they are rational numbers.

Terminating Decimal Numbers
Number Fraction
0.5 1/2
1.53 153/100
52.372 13 093/250
Pop Quiz

Identifying Types of Decimals

Determine whether the given number is a repeating decimal, a terminating decimal, or neither.

Discussion

Converting a Fraction Into a Repeating Decimal Number

Fractions can be written as decimal numbers using long division. However, when the denominator of a fraction has a factor other than 2 or 5, the quotient will be a repeating decimal number. Consider the following fraction. 7/15 It is possible to write this fraction as a decimal in two steps.

1
Use Long Division
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Divide the numerator by the denominator using long division.

Dividing 7 by 15

After some point, the remainder does not become zero and a particular digit or set of digits in the quotient keeps repeating. This means that the quotient is a repeating decimal number.

2
Use Bar Notation to Write the Repeating Decimal
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We found that the fraction is a repeating decimal number. 7/15 = 0.4666... The digit that repeats is 6, so a bar is drawn over that digit. 7/15 = 0.46
Example

Favorite Movie Genres

A survey asked students what kinds of movies they like. The table shows the results.

Genre Number of Students
Action 6
Animation 12
Comedy 4
Science Fiction 5
a

What fraction of the students prefer animation?

b

What fraction of the students prefer comedy?

Hint

a

How many students answered the survey? Write a fraction using the number of students that prefer animation and the total number of the students surveyed. Convert the fraction into a decimal number.

b

Write a fraction using the number of students that prefer comedy and the total number of the students surveyed. Convert the fraction into a decimal number.

Solution

a

Let's start by finding the total number of students who participated in the survey.

Genre Number of Students
Action 6
Animation 12
Comedy 4
Science Fiction 5
Total 27

We can see that 27 students responded to the survey. Of these students, 12 prefer animation. We can represent this as a fraction by writing the part, the 12 students who like animation, over the whole, the 27 students surveyed. 12/27 Since the given options are decimals, let's converted into a decimal number using long division. We will continue until the remainder is 0 or a pattern appears.

Dividing 12 by 27

It appears that the remainder will always repeat — in other words, the digit 4 in the quotient will always repeat. This means that the division resulted in a repeating decimal number. 12/27 = 0.444... The same value can also be written by drawing a bar over the repeating digit. 12/27 = 0.4 The answer is A.

b

From the table, we can see that four students like the comedy genre. This means that 427 of the students surveyed prefer comedies. Let's use long division again to rewrite it as a decimal. We will continue the division until the remainder is 0 or a pattern appears.

Dividing 4 by 27

It looks like the remainder matches the difference after our first subtraction. Since the subtraction pattern will restart, more than one digit repeats in the quotient. 4/27 & = 0. 148148... Put a bar above those three digits to indicate the repetition. This repeating decimal corresponds to option C. 4/27 & = 0.148

Discussion

Converting a Repeating Decimal Number Into a Fraction

Terminating decimal numbers can be converted to fractions by dividing the number by 10^n, where n is the number of decimal places. Terminating Decimal & & Fraction 0.56 & & 56/100 However, it is impossible to count the number of decimal places for repeating decimals because they have an infinite number of digits after the decimal point. Repeating Decimal & & Fraction 0.083 & & ? Even though they have an infinite number of digits, these numbers can be expressed as fractions. Follow these five steps to do so.

1
Assign a Variable for the Repeating Decimal
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Let x be a variable that represents the given repeating decimal number. We can write an equation by setting x equal to this number. Consider 0.083 as an example. x = 0.083
2
Count the Number of Repeating Digits and Let It Be n
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For the given decimal, the repeating digit is 3. x = 0.083 There is only one digit with a bar above it, so n= 1.
3
Multiply the Repeating Decimal by 10^n
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Since there is one repeating digit, multiply each side of the equation by 10^1, or 10.

x = 0.083
x * 10 = 0.083 * 10
10x = 0.83

4
Subtract the Equation in Step 1 From the Equation in Step 3
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Now subtract the original equation from the equation obtained in Step 3. cl & 10x = 0.83 - & x = 0.083 Writing the decimals without using bar notation can make subtraction easier to understand. cl & 10x = 0.83333 ... - & x = 0.08333 ... & 9x = 0.75000 ... The zeros can be ignored. We found that 9x is equal to 0.75.

5
Solve for the Variable and Express It as a Fraction in Its Simplest Form
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Finally, solve the equation for x.

9x = 0.75
9x/9 = 0.75/9
9x/9 = 0.75/9
x = 0.75/9

Start by eliminating the decimal in the numerator to simplify the fraction. This can be done by multiplying the numerator and denominator by 10^2 because the numerator has 2 decimal places.

x = 0.75/9
x = 0.75 * 100/9* 100
x = 75/900

Since the greatest common factor of 900 and 75 is 75, we will simplify the fraction by dividing both the numerator and denominator by 75.

x = 75/900
x = 75/ 75/900 / 75
x = 1/12

We found that x is equal to 112. Remember that x is also equal to 0.083. x = 0.83 and x = 1/12 Therefore, by the Transitive Property of Equality, the fraction is equal to the given repeating decimal number. 0.083=1/12

Example

Which Two Genres Did Vincenzo Choose?

A survey asked students what kinds of movies they like. The table shows the results.

Genre Number of Students
Action 6
Animation 12
Comedy 4
Science Fiction 5

Vincenzo selected two genres at random. He used a calculator to divide the number of students who prefer the two genres by the total number of students surveyed.

His calculator showed the result as 0.629629....

a

Write the result as a fraction.

b

Which two movie genres did Vincenzo choose?

Hint

a

Let x represent the repeating decimal and write an equation. How many repeating digits are there in the decimal number? Multiply the equation by a power of 10.

b

What is the numerator of the fraction we found in Part A? Determine which of the two types gives us a sum equal to this numerator.

Solution

a

The number shown on the calculator is a repeating decimal number because some digits after the decimal point repeat themselves.

0. 629 629... ⇔ 0.629 There are five steps to write this repeating decimal number as a fraction.

  1. Assign a variable to represent the repeating decimal.
  2. Count the number of repeating digits and call it n.
  3. Multiply the repeating decimal by 10^n.
  4. Subtract the equation in Step 1 from the equation in Step 3.
  5. Solve for the variable and express it as a fraction in its simplest form.

Step 1

Let x be a variable that represents the given repeating decimal number. Write an equation by setting x equal to 0.629. x = 0.629

Step 2

The repeated sequence of digits in the decimal is 629. There are three repeating digits, so n= 3.

Step 3

Since n=3, multiply each side of the equation by 10^3, or 1000.

x = 0.629
x * 1000 = 0.629 * 1000
1000x = 629.629

Step 4

Now we subtract the original equation from the equation we created in Step 3. cr & 1000x = 629.629 [0.2em] - & x = 0.629 Consider the expanded forms of the numbers when performing this subtraction. cr & 1000x = 629.629629 ... [0.2em] - & x = 0.629629 ... & 999x = 629.000000 ... Since we can ignore the zeros after the decimal place, we get that 999x is equal to 629.

Step 5

Let's solve our equation for x.

999x = 629
999x/999 = 629/999
999x/999 = 629/999
x = 629/999

We can check whether this fraction can be simplified by finding the greatest common factor (GCF) of the numerator and denominator. 629 = 17 * 37 999 = 3^3 * 37 The GCF(629,999) is 37. Finally, simplify the fraction by dividing both numerator and denominator by 37.

x = 629/999
x = 629/37/999 /37
x = 17/27

As a result, x is equal to 1727, which is equal to the given repeating number. 0.629=17/27

b

In Part A, it was found that 0.629 is equivalent to 1727. Notice that the denominator is the same as the total number of students surveyed, so 1727 should be the fraction that Vincenzo entered into the calculator. The numerator of this fraction is the sum of the students who prefer the two specific genres.

Now let's take a look at the table and determine which of the two genres gives a sum equal to 17.

Genre Number of Students
Action 6
Animation 12
Comedy 4
Science Fiction 5

The number of students who prefer animation and science fiction is 17, so Vincenzo selected the animation and science fiction genres.

Pop Quiz

Practice Converting Between Fractions and Repeating Decimals

Write the given fraction as a repeating decimal and vice versa.

Randomly generated fraction or decimal number

Extra

How to Submit a Repeating Decimal
If the answer is a repeating decimal, do the following to submit the answer.

  1. Enter the non-repeating part of the number, including the decimal point, into the first box.
  2. Enter the repeating part into the second box.

Here are some examples. Repeating Decimal & & How to Submit 0.409 & ⇒ & 0.4 09 [0.6em] 0.9 & ⇒ & 0. 9


Example

Was Archimedes Correct?

Greek mathematician and physicist Archimedes made many contributions to mathematics. One of his contributions was a method he used to approximate the value of π. π = 3.141592 ... Although this number cannot be expressed exactly as a ratio of two integers, Archimedes claimed that π was between 3 17 and 3 1071. Was he correct?

Hint

Write the mixed numbers as decimals.

Solution

Archimedes believed that π was between two rational numbers, 3 17 and 3 1071. π = 3.141592... To check whether he was correct, let's write both numbers as decimals. Since the first six decimal places of π are given, we only need to calculate our decimals up to six places to compare them.

Rewriting 3 17

First, rewrite 3 17 as an improper fraction.

3 17
Write mixed number as a fraction
3 * 7 + 1/7
21 + 1/7
22/7

Next, use long division to divide the numerator by the denominator of the fraction.

Dividing 22 by 7

The mixed number is equal to 3.142857.... 3 17 & = 3.142857...

Rewriting 3 1071

Let's follow the same steps as before to write 3 1071 as a decimal. We start by rewriting the mixed number as an improper fraction.

3 1071
Write mixed number as a fraction
3 * 71 + 10/71
213 + 10/71
223/71

Next, divide the numerator 223 by the denominator 71. Calculate to 6 decimal places.

Dividing 223 by 71

We found that 3 1071 is equal to 3.140845 .... 3 1071 = 3.140845...

Comparing the Numbers

Now we can compare the three numbers. 3 17 &= 3.142857... 3 1071 &= 3.140845... π &= 3.141592... Notice that the first three digits of the numbers are all the same. 3 17 &= 3.142857... 3 1071 &= 3.140845... π &= 3.141592... This means that the digits in the thousandths place must be compared. 3 17 &= 3.14 2857... 3 1071 &= 3.14 0845... π &= 3.14 1592... The number with the greatest digit in the thousandth place is the greatest, and the number with the least digit is the least. 3.140845... < 3.141592... < 3.142857... ⇓ 3 1071 < π < 3 17 Therefore, π is between 3 1071 and 3 17. Archimedes was correct. The answer is B.

Extra

Archimedes's Method
Archimedes's technique to approximate π was quite simple — he drew regular polygons inside and outside of a circle. He used a circle with a diameter of 1 because its circumference is π.

The circumference of the circle would be somewhere between the perimeters of the polygons. Archimedes calculated the perimeters of the polygons using geometric arguments.

His final estimate for π use a shape with 96 sides. His calculations gave a range between 3 1071 and 3 17. 3 1071 < π < 3 17 Of course, he found this result with pen and paper, without the help of a computer or calculator. Amazing!

Closure

Why Is π So Special?

This lesson presented two types of decimal numbers, terminating and repeating decimals, and methods for converting both to and from fractions. Now comes a tough question! What about decimal numbers that have an infinite number of places but do not repeat? These decimals are called irrational numbers. Take, for example, π. π = 3.141592 ... We do not see any repeating patterns in this decimal number, so it has no end. The number is irrational. Here are a few other commonly used irrational numbers.

Some Irrational Numbers
Euler's number, e 2.718281 ...
Golden ratio, ϕ 1.618033 ...
sqrt(2) 1.414213 ...

Irrational numbers cannot be converted into rational numbers. This diagram shows how decimal numbers are classified.



Terminating and Repeating Decimals
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