Computing

15

Chapter 3

Rational and

Irrational Numbers

3.1 APPEARANCE OF FRACTIONS

Counting is the simplest and most fundamental operation using numbers.

However, just counting was insucient and inconvenient for people even

in ancient civilizations. People at some early stage of their civilizations

realized that arithmetic operations such as addition, subtraction, multi-

plication, and division were necessary and very important. It is probably

more than 4000 years ago that people in Egypt, Mesopotamia, and other

ancient civilized regions could already manipulate arithmetic operations

using their number systems, although it is dicult to specify when and

where people rst discovered such techniques for calculations.

Among the four arithmetic operations, multiplication and division

were dicult compared with addition and subtraction. People invented

doubling and halving operations a long time ago. ese and similar opera-

tions were convenient for their lives, and made multiplication and division

easier. Consequently, the fractions 1/2, 1/4, 1/8, 1/16, and so on became

commonly used numbers in addition to natural numbers. Furthermore,

the use of reciprocals of integers, such as 1/2, 1/3, 1/4, 1/5, 1/6, and so

on, also became common and of great importance. e reciprocal of each

nonzero natural number is called a unit fraction. ey noticed that multi-

plications of unit fractions by integers were also useful for calculations. In

this way, fractions appeared in ancient civilizations [7].

16 ◾ Computing

Ancient Egyptians used special symbols representing fundamental

unit fractions and some other fractions (e.g., 2/3). ese symbols appeared

in several mathematical tablets and papyri found in Egypt [10]. Akhmim

wooden tablets and Cairo wooden tablets are two ancient Egyptian doc-

uments that contain, for example, descriptions about multiplications

by fractions 1/3, 1/7, 1/10, 1/11, and 1/13. e following calculation of

fractions also appeared in the tablets:

1/2 + 1/4 + 1/8 + 1/16 + 1/64 + 5/320 = 1

Historians suggest that these tablets were probably inscribed at the

beginning of the Egyptian Middle Kingdom around 1950 BC [3]. ese

are now held in the Egyptian Museum in Cairo.

In the British Museum in London, the Rhind Papyrus (also called Rhind

Mathematical Papyrus) is displayed. It is approximately 5 m long and 33 cm

wide, one of the oldest existing texts of Egyptian mathematics. A Scottish

lawyer, Alexander H. Rhind (1833–1863), purchased it in 1858 in Egypt

[8]. It was named the Rhind Papyrus aer him. It was copied by a scribe

named Ahmes around 1650 BC. In the rst paragraph of the papyrus,

Ahmes presents that it is copied from an ancient copy made during the

12th dynasty of Upper and Lower Egypt (c. 1985–1795 BC) [3, 8].

e rst part of the Rhind Papyrus contains a list of the fractions 2/n

for odd n from 3 to 101. e following are examples in the list:

2/3 = 1/2 + 1/6

2/5 = 1/3 + 1/28

2/7 = 1/4 + 1/28

2/9 = 1/6 + 1/18

2/15 = 1/10 + 1/30

2/101 = 1/101 + 1/202 + 1/303 + 1/606

e second and third parts of the Rhind Papyrus consist of geometry

problems, and 84 problems with the solutions, respectively. Below is an

example in the third part:

Rational and Irrational Numbers ◾ 17

Problem 3.1

Let the sum of 2/3 and 1/10 of an unknown quantity be 10. Calculate

the unknown quantity.

SOLUTION

Using modern notation, (2/3 + 1/10)x = 10, where x is the unknown

quantity. en x = 300/23.

e Moscow Papyrus (also called Moscow Mathematical Papyrus) is

now held in the Puskin State Museum of Fine Arts in Moscow [9]. It is

approximately 5.5 m long and 3 ~ 7.7 cm wide, one of the oldest Egyptian

mathematical texts in existence. e Moscow Papyrus probably dates

to the 11th dynasty of Upper and Lower Egypt (c. 1850 BC). It contains

25 problems of arithmetic, algebra, and geometry. e following are

examples of the problems:

Problem 3.2

Let the sum of 1 and a half of an unknown quantity and 4 be 10.

Calculate the unknown quantity.

SOLUTION

Using modern notation, (1 + 1/2)x + 4 = 10, where x is the unknown

quantity. en x = 4.

Problem 3.3

Let 1/2 + 1/4 of the square of an unknown quantity be 12. Calculate

the unknown quantity.

SOLUTION

Using modern notation, (3/4)(3/4)x

2

= 12(3/4), where x is the unknown

quantity. en x = 4.

3.2 RATIONAL NUMBERS

e set of rational numbers is usually denoted by Q, and the elements

of this set have the property that they can be represented as a ratio

of two integers (hence the name rational). ese numbers are, in a