ncert-textbook-for-class-10-maths-chapter-1 (1).pdf

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REAL NUMBERS
12 Introduction

InClass IX, you began your exploration of the world of real numbers and encountered
irrational numbers. We continue our discussion on real numbers in this chapter. We
begin with two very important properties of postive integers in Sections 1.2 and 1.3,
:amely the Euclid’s division algorithm and the Fundamental Theorem of Arithmetic.

Euclid* division algorithm, asthe name suggests; has to do with divisbiliy of
integers. Stated simply. it says any positive integer a can be divided by another positive
integer bin such a way tha it leaves a remainder rthat is smaller than b. Many of you
probably recognise this as the usual long division process. Although this results quite
easy o state and understand, thas many applications related tothe ivisibiliy properties
of integers. We touch upon a few of them, and use it mainly to compute the HCF of
two positive integers,

“The Fundamental Thcore fArimetic, onthe other hand, has o do something
‘with multiplication of postive imegers Yu already know that every composite number
Can be expressed a a prodictof primes in a unique way —this important fact is he
Fundamental Theorem oC Arithmetic. Again, while ts resultat easy to tate and
understand, has some very deep and significant applications inthe fed of mathematics.
We use the Fundamental Theorem of Arithmetic fortwo main applications Fis, we
use itt provetheimationalityof many of the numbers you studied in Class IX, such as
V3, and VF: Second, we apply this thcorem to explore when exactly the decimal

8
factorisation ofthe denominator

14 when it is non

<span of ional number, ay 241 +0, tem
termining een. Wedo soy ak
qof 2. You will see that the prime factorisation of q will completely reveal the nature

atthe prin

ofthe decimal expansion of 2.

q
So let us begin our exploration.

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2. 2 Marie

12 The Fundamental Theorem of Arithmetic
In your earlier classes, you have seen that any natural number can be written as a
product of its prime factors For instance, 2= 2, 4=2 x 2, 1 = 23, and soon.
Now; let us try and look at natural numbers from the other direction. Tha is, can any
natural number be obtained by multiplying prime numbers? Let us see.

Take any collection of prime numbers, say 2, 3, 7, 11 and 23. If we multiply
some or all of these numbers, allowing them to repeat as many times as we wish
we can produce a large collection of positive integers (In fact, infinitely many)
Let us lista few

7112351771 Bx Te <= 5313
2x 3x7 XM x 23= 106% 3x 728232
Bx 3x7 «11x23=21252

and soon,

Now, et us suppose your collection of primes includes all he possible primes

‘What is your guess about the size of this collection? Does it contin only a finite

numberof integers, or infinitely many’? Infct, her ae infinitely many primes. So,

if we combine all these primes in all possible ways. we will get an infinite

collection of umber, all

the primes and all possible [32760]

products of primes. The

question is - can we

produce all the composite

numbers this way? What

do you think? Do you

mann 12] [

composite number which

is not the product of 7 [10]

powers of primes?

Before we answer this

Jet us factoise positive en nel

integers, that i, do the

opposite of what we have

done so fr 3 El
We ae going to use

the tort with which

vou ar al familar Letus

take some large number

say, 32760, and factorise a
itas shown

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3. Rea Nues 3

So we have factorised 32760 as 2 » 2 » 2 » 3» 3 x 5 x 7 « 13 asa product of
primes, ie, 32760 = 2 x 3°» $7 x 13 as a product of powers of primes, Let us try
‘another number, say, 123456789. This can be written as 3° » 3803 » 3607, Ofcourse,
you have to check that 3803 and 3607 ate primes! (Try it out for several other natural
numbers yourself) This leads us toa conjecture that every composite number can be
‘written as the product of powers of primes. In fact, this statement is true, and is called
the Fundamental Theorem of Arithmetic because of its basic crucial importance
tothe study of integers, Let us now fomnally stat this theorem.

‘Theorem 1.1 (Fundamental Theorem of Arithmetic) : Every composite
number can be expressed (factorised) as a product of primes, and this factorisation
is nique, apart from the order in which the prime factors occur

An equivalent version of Theorem 1.2 was probably
first recorded as Proposition 14 of Book IX inEnclid'

Elements, before it came to be known as the

Fundamental Theorem of Arithmetic: However, the

first comect proof was given by Carl Friedrich Gauss

in his Disquisitiones Arithmeticae

‘Carl Friedrich Gaussis often rferredto asthe ‘Prince

‘of Mathematicians’ andis considered one of he three

greatest mathematicians of all time, along with

Archimedes and Newton, He has made fundamental Carl Friedrich Gauss
‘contributions to both mathematics and science. (1777 ~ 1855)

The Fundamental Theorem of Arithmetic says that every composite number can
be factorised as a product of primes. Actually it says more. It says that given any
composite number it can be factorised as a product of prime mumbers in a ‘unique’
way. except for the order in which the primes oceur. That is, given any composite
number there is one and only one way to wie itas a product of primes, as long as we
‘ae not particular about the order in which the primes occur. So, for example, we
regard 2 3 %5 * Tasthe same as 3 «5 = 7 «2, or any other possible order in which
these primes are written. This fact is also stated in the following form:

The prime factorisation of a natural monber is unique, except for the order
of is factors.

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4. 4 Mamans

In general, given a composite number x, we factorise it as x = pp, Pas Where
Py Ps P, are primes and written in ascending order, ie, p, < P,
S... Sp, lf we combine the same prime

32760= 22% 2%3%3x5%7% 13

‘Once we have decided that the order will be ascending, then the way the number.
is factorised is unique.

‘The Fundamental Theorem of Arithmetic has many applications, both within
‘mathematics and in other fields. Let us look at some examples.

„we will get powers of primes. For example,
x BXS XTX 13

Example 1: Consider the numbers 4, where n isa natural number. Check whether
there is any value of n for which 4* ends withthe digit zero.

Solution «Ifthe number 4, for any n, were to end with the digit zero, then it would be
divisible by 5. That is, the prime factorisation of 4* would contain the prime 5. This is
ot possible because 4" = (2); so the only prime in the factorisation of 4" is 2. So, the
‘uniqueness of the Fundamental Theorem of Arithmetic guarantees that there are no
other primes inthe factorisation of 4° So, there is no natural number n for which 4%
ends with the digit zero.

You have already learnt how to find the HCF and LCM of two positive integers
using the Fundamental Theorem of Arithmetic in carier classes, without realising it!
‘This method is also called the prime factorisation method. Let us reall this method
through an example.

Example 2 : Find the LEM and HCE Of 6 and 20 by the prime factorisation method.
Solution + We have : 6223! and 20=2%2x5=2x5!

‘You can find HCF(6, 20) = 2 and LCM(6, 20) = 2 x 2 x 3 x 5 = 60, as done in your
carer clases,

Note that HCF(6, 20) = 2!

oduct of the smallest power of each common
prime factor in the numbers.

rroduct of the greatest power of each prime factor,
involved in the numbers.

PE

LCM (6, 20)

From the example above, you might have noticed that HCF(6, 20) x LCM(G, 20)
x 20. In fact, we can verify that for any two positive integers a and b,
HCE (a, 6) x LCM (a, b) = a x b. We can use this result to find the LCM of two
positive integers, if we have already found the HCF of the two positive integers.

Example 3: Find the HCF of 96 and 404 by the prime factorisation method. Hence,
find their LCM.

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5. Rea Nora 5

Solution : The prime factorisation of 96 and 404 gives
96 =2° x3, 404 =2°x 101
‘Therefor, the HCF of these two integers is 2

96% 404
TCR 96, 404)

96x 404

Also, LCM (96, 404) = = 9696

Example 4 + Find the HCF and LCM of 6, 72 and 120, using the prime factorisation
method.

Solution + We have :
6=2x3, 12 =2'x 3%, 120022) x3 x5

Here, 2! and 3! are the smallest powers of the common factors 2 and 3, respectively

So, HCF (6, 72, 120) = 2! x 3 6

2,3 and 5! are the greatest powers ofthe prime factors 2,3 and 5 respectively
involved in the three numbers.

So, LCM (6, 72, 120) «23x # x 5! 2360

Remark + Notice, 6 x 72 x 120% HOF (6, 72, 120) x LCM (6, 72, 120). So, the
product of three numbers is not equal to the product oftheir HCF and LCM.

EXERCISE 1.1
1. Express each number as a prodie fits prime factors:
om Go 156 CES iv) ss a

2. Find the LCM and HCE of the following pairs of integers and verify that LCM x HCI
product ofthe two numbers.

© 26andoı i) StOand92 iD Bands
3. Find the LEM and HCF of the following integers by applying he prime factorisation
method
© 12)Sand21 Ci) 17,23and29 (i) 8,9and 28

Given that HCF 806,657) =9, find LCM 306.657)
‘Check whether" can end with the digi O for any natural number
Explain why 7x 1113413 and 7x6x5x4x3x2x 14 Sar composite numbers,

‘There isa circular path around spot fiel Sonia takes 18 minutes o drive one round.
‘ofthe Feld, while Ravi takes 12 minutes for the same, Suppose they both start atthe

sons

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6. 6 Marie

same point and at the sat

will hey meet again atthe start

time, and go inthe same direction. Afterhow many minutes
in?

13 Revisiting Irrational Numbers
In Class IX, you were introduced t imational numbers and many oftheir properties.
You studied abou their existence and how the rationals and the iationas together
made up the real numbers. You even studied howto locate rations on the number
line, However, we did not prove tht they were imationals In ths section, we will
mrovethat V3, V3, 5 and.ingenerl, „p isimational, where pis a prime, One of
the theorems, we use in ou proof is the Fundamental Theorem of Ariane,
Recall, amumber ‘is called irrational cannot be writen inthe fon 2.

where p and q are integers and q = 0. Some examples of rational numbers, wth
‘which you are already famili, ae

E68. Zoomen se

Before we prove that /3 is iraliOnal we need the folowing theorem, whose
roof is based onthe Fundamental Theorem of Arithmetic

haie 12 2 Let p be a prime number If p divides a, then p divides a, where
a is a positive integer

“Proof Let the prime factorsation of a be as follows

1D: Py WHETE Pp. Lane primes, not necessarily distinct
‘Therefore, a = (pp, PDP). P,) = PIPL Pi

Now, we are given that p divides a Therefore, from the Fundamental Theorem of
Arithmetic, it follows that ps one of the prime factors of a, However, using the
‘miqueness par ofthe Fundamental Theorem of Arithmetic, we realise thatthe only
prime factors of ae. D» PS0 pis OME Of. Pe » Py

Now, since @=p;P,.. .p, p divides a
‘We are iow ready to give a proof that „3 is imation

‘The proof is based on a technique called “proof by contradiction’. (This technique is
discussed in some detail in Appendix 1).

core 13 4/2 is érrational
Proof: Let us assume, to the contrary, that „/2 is rational

© Not from the examination pont of view.

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7. ReauNuusers 7

So, we can find integers rand s (= 0) such that 3

‘Suppose rand shave ac

nmon factor other than 1. Then, we divide by the common.

factor to get /Z= ©, where a and bare coprime

Squaring on both sides and rearranging, we get 20° = a. Therefore, 2 divides a
Now by Theorem 13, it follows that 2 divides a

So, we can write a= 2e for some integer c
‘Substituting for a, we get 26 = 4e, that is, D
‘This means that 2 divides Band so 2 divides b (again using Theorem 1.3 with p = 2)
‘Therefore, a and b have at least 2 as a common factor.

But this contradicts the fact that a and b have o common factor other than 1
‘This contradiction has arisen because of our incorrect assumption that J isrational
So, we conclude that /3 is imational

Prove that 3 is tational
Solution + Lets assume, tothe contrary, tha J is rational

Example 5

Main e an inet amd ech tt 5 = €
Sippe nd ave comin aor te an 1, en ve can de y
cotton fc stn th te opine

So, bV3 =a

‘Squaring on both sides, and rearranging, we get 3!
"hee re y, ny These 13, ta ia de
ws

So. we canis 3 o omega

Sin fora wege 29 Uat B=

mena eve 3, an bilo vil y (ao Teen 13
with p=3),

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8. 8 Mamans

Therefore, a and b have at least 3 as a common factor.
But his contradicts the fact hat a and b are coprime
‘This contradiction has arisen because of our incorrect assumption that /3 is rational
So, we conclude that JF is rational.

In Class IX, we mentioned that

e the sum ordif

nce of a rational and an irrational number is irrational and

e the product and quotient of a non-zero rational and irrational number is
irracional,

We prove some particular cases here,
Example 6 + Show that 5 — 3 is irrational,
Solution + Let us assume, to the contrary, that $= 3 is rational,

‘That is, we can find coprime a and b (b #0) such that 5— 3

Therefore, 5 — = =

Rearranging this equation, we get Ya = 5 - 22 3274

Ca
Sincea and Daniel, et 52 ion and so is ation

But his contradict

the fact that 7 HS rational.

- Bis

‘This contradiction has arisen because of our incorrect assumption that 5
rational

So, we conclude that 5 — V3 is irrational,

Example 7 yShow that 3V2 is irrational.
‘Solution + Letus assume, to the contrary, that 3/2 is rational.

That is, we can find cope

ne a and b (00) such that 5/2 =
El
Since 3, a nd bare integers, 3, is rational, and so 2 is rational.

Rearanging, we get V3

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‘But this contradicts he fact that 3 is iraiona.

So, we conclude that 3/3 is irrational,

EXERCISE 1.2
1. Prose that VF isimotional
2. Prove that 3 4 2,8 is iational

3. Prove thatthe following ae rationals

os & NB W643

14 Summary
In this chapter, you have studied the following points?
1. The Fundamental Theorem of Arithmetic

Every composite number canbe expressed (Fatorised) ava product of primes, and this
factoristion is unique, apart from the order in which the prime factors occur.

2. pis prime and p divides a hen p divides zs where ais positive integer

3. Toprove that JB, VF are rationals

ANog# ro TUE READER

‘You have seen th

HCF (p.9, )xLCM(p. 4.1) # pq xr, where p, q, rare positive integers
(see Example): However, the following results hold good for three numbers

peg-r-HCF(p. 4.1)
HCF(p, q) -HCF(g.r) - HCF(p,n)

Dar CM. qm
LCM(p. 4) LOM, 7) LMC)

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