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Class X Chapter 1 – Real Numbers Maths

Exercise 1.1
Question 1:
Use Euclid’s division algorithm to find the HCF of:

Answer:
(i) 135 and 225

Since 225 > 135, we apply the division lemma to 225 and 135 to
obtain
225 = 135 × 1 + 90

Since remainder 90 ≠ 0, we apply the division lemma to 135 and 90 to

obtain
135 = 90 × 1 + 45
We consider the new divisor 90 and new remainder 45, and apply the
division lemma to obtain
90 = 2 × 45 + 0

Since the remainder is zero, the process stops.

Since the divisor at this stage is 45, Therefore,
the HCF of 135 and 225 is 45. (ii)196 and
38220

Since 38220 > 196, we apply the division lemma to 38220 and 196 to
obtain
38220 = 196 × 195 + 0
Since the remainder is zero, the process stops.
Since the divisor at this stage is 196,

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Class X Chapter 1 – Real Numbers Maths

Therefore, HCF of 196 and 38220 is 196.

(iii)867 and 255

Since 867 > 255, we apply the division lemma to 867 and 255 to
obtain
867 = 255 × 3 + 102

Since remainder 102 ≠ 0, we apply the division lemma to 255 and 102
to obtain
255 = 102 × 2 + 51
We consider the new divisor 102 and new remainder 51, and apply the
division lemma to obtain
102 = 51 × 2 + 0
Since the remainder is zero, the process stops.
Since the divisor at this stage is 51,
Therefore, HCF of 867 and 255 is 51.
Question 2:

Show that any positive odd integer is of the form , or , or

, where q is some integer.

Answer:

Let a be any positive integer and b = 6. Then, by Euclid’s algorithm, a

= 6q + rfor some integer q ≥ 0, and r = 0, 1, 2, 3, 4, 5 because 0 ≤ r
< 6.

Therefore, a = 6q or 6q + 1 or 6q + 2 or 6q + 3 or 6q + 4 or 6q + 5
Also, 6q + 1 = 2 × 3q + 1 = 2k1 + 1, where k1 is a positive integer

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Class X Chapter 1 – Real Numbers Maths

6q + 3 = (6q + 2) + 1 = 2 (3q + 1) + 1 = 2k2 + 1, where k2 is an

integer
6q + 5 = (6q + 4) + 1 = 2 (3q + 2) + 1 = 2k3 + 1, where k3 is an
integer
Clearly, 6q + 1, 6q + 3, 6q + 5 are of the form 2k + 1, where k is an
integer.
Therefore, 6q + 1, 6q + 3, 6q + 5 are not exactly divisible by 2.
Hence, these expressions of numbers are odd numbers.
And therefore, any odd integer can be expressed in the form 6q + 1,
or 6q + 3,
or 6q + 5
Question 3:
An army contingent of 616 members is to march behind an army band
of 32 members in a parade. The two groups are to march in the same
number of columns. What is the maximum number of columns in
which they can march?
Answer:

HCF (616, 32) will give the maximum number of columns in which
they can march.
We can use Euclid’s algorithm to find the HCF.
616 = 32 × 19 + 8
32 = 8 × 4 + 0
The HCF (616, 32) is 8.
Therefore, they can march in 8 columns each.

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Class X Chapter 1 – Real Numbers Maths

Question 4:

Use Euclid’s division lemma to show that the square of any positive
integer is either of form 3m or 3m + 1 for some integer m.
[Hint: Let x be any positive integer then it is of the form 3q, 3q + 1 or
3q + 2. Now square each of these and show that they can be rewritten
in the form 3m or 3m + 1.]
Answer:

Let a be any positive integer and b = 3.

Then a = 3q + r for some integer q ≥ 0
And r = 0, 1, 2 because 0 ≤ r < 3
Therefore, a = 3q or 3q + 1 or 3q + 2
Or,

Where k1, k2, and k3 are some positive integers

Hence, it can be said that the square of any positive integer is either of
the form 3m or 3m + 1.
Question 5:

Use Euclid’s division lemma to show that the cube of any positive
integer is of the form 9m, 9m + 1 or 9m + 8. Answer:

Let a be any positive integer and b = 3

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a = 3q + r, where q ≥ 0 and 0 ≤ r < 3

Therefore, every number can be represented as these three forms.

There are three cases.
Case 1: When a = 3q,

Where m is an integer such that m =

Case 2: When a = 3q + 1,

a3 = (3q +1)3
a3 = 27q3 + 27q2 + 9q + 1
a3 = 9(3q3 + 3q2 + q) + 1
a3 = 9m + 1
Where m is an integer such that m = (3q3 + 3q2 + q)
Case 3: When a = 3q + 2,

a3 = (3q +2)3
a3 = 27q3 + 54q2 + 36q + 8
a3 = 9(3q3 + 6q2 + 4q) + 8
a3 = 9m + 8
Where m is an integer such that m = (3q3 + 6q2 + 4q)
Therefore, the cube of any positive integer is of the form 9m, 9m + 1,
or 9m + 8.

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Class X Chapter 1 – Real Numbers Maths

Exercise 1.2
Question 1:
Express each number as product of its prime factors:

Answer:

Question 2:

Find the LCM and HCF of the following pairs of integers and verify that
LCM × HCF = product of the two numbers.

Answer:

Hence, product of two numbers = HCF × LCM

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Hence, product of two numbers = HCF × LCM

Hence, product of two numbers = HCF × LCM

Question 3:

Find the LCM and HCF of the following integers by applying the prime
factorisation method.

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Answer:

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Class X Chapter 1 – Real Numbers Maths

Question 4:
Given that HCF (306, 657) = 9, find LCM (306, 657).
Answer:

Question 5:
Check whether 6n can end with the digit 0 for any natural number n.
Answer:

If any number ends with the digit 0, it should be divisible by 10 or in

other words, it will also be divisible by 2 and 5 as 10 = 2 × 5 Prime
factorisation of 6n = (2 ×3)n
It can be observed that 5 is not in the prime factorisation of 6n.
Hence, for any value of n, 6n will not be divisible by 5.

Therefore, 6n cannot end with the digit 0 for any natural number n.
Question 6:
Explain why 7 × 11 × 13 + 13 and 7 × 6 × 5 × 4 × 3 × 2 × 1 + 5 are
composite numbers.
Answer:

Numbers are of two types - prime and composite. Prime numbers can
be divided by 1 and only itself, whereas composite numbers have
factors other than 1 and itself.

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It can be observed that

7 × 11 × 13 + 13 = 13 × (7 × 11 + 1) = 13 × (77 + 1)
= 13 × 78
= 13 ×13 × 6

The given expression has 6 and 13 as its factors. Therefore, it is a

composite number.
7 × 6 × 5 × 4 × 3 × 2 × 1 + 5 = 5 ×(7 × 6 × 4 × 3 × 2 × 1 + 1)
= 5 × (1008 + 1)
= 5 ×1009
1009 cannot be factorised further. Therefore, the given expression has
5 and 1009 as its factors. Hence, it is a composite number.
Question 7:
There is a circular path around a sports field. Sonia takes 18 minutes
to drive one round of the field, while Ravi takes 12 minutes for the
same. Suppose they both start at the same point and at the same
time, and go in the same direction. After how many minutes will they
meet again at the starting point?
Answer:

It can be observed that Ravi takes lesser time than Sonia for
completing 1 round of the circular path. As they are going in the same
direction, they will meet again at the same time when Ravi will have
completed 1 round of that circular path with respect to Sonia. And the
total time taken for completing this 1 round of circular path will be the
LCM of time taken by Sonia and Ravi for completing 1 round of circular
path respectively i.e., LCM of 18 minutes and 12 minutes.

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18 = 2 × 3 × 3
And, 12 = 2 × 2 × 3
LCM of 12 and 18 = 2 × 2 × 3 × 3 = 36

Therefore, Ravi and Sonia will meet together at the starting pointafter
36 minutes.

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Class X Chapter 1 – Real Numbers Maths

Exercise 1.3
Question 1:

Prove that is irrational.

Answer:

Let is a rational number.

Therefore, we can find two integers a, b (b ≠ 0) such that

Let a and b have a common factor other than 1. Then we can divide
them by the common factor, and assume that a and b are co-prime.

Therefore, a2 is divisible by 5 and it can be said that a is divisible by 5.

Let a = 5k, where k is an integer

This means that b2 is divisible by 5 and hence, b is divisible

by 5.
This implies that a and b have 5 as a common factor.
And this is a contradiction to the fact that a and b are co-prime.

Hence, cannot be expressed as or it can be said that is

irrational.
Question 2:

Prove that is irrational.

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Answer:

Let is rational.
Therefore, we can find two integers a, b (b ≠ 0) such that

Since a and b are integers, will also be rational and

therefore, is rational.

This contradicts the fact that is irrational. Hence, our assumption

that is rational is false. Therefore, is irrational.

Question 3:
Prove that the following are irrationals:

Answer:

Let is rational.
Therefore, we can find two integers a, b (b ≠ 0) such that

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is rational as a and b are integers.

Therefore, is rational which contradicts to the fact that is

irrational.

Hence, our assumption is false and is irrational.

Let is rational.
Therefore, we can find two integers a, b (b ≠ 0) such that

for some integers a and b

is rational as a and b are integers.

Therefore, should be rational.

This contradicts the fact that is irrational. Therefore, our assumption

that is rational is false. Hence, is irrational.

Let be rational.

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Therefore, we can find two integers a, b (b ≠ 0) such that

Since a and b are integers, is also rational and hence, should

be rational. This contradicts the fact that is irrational. Therefore,

our assumption is false and hence, is irrational.

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Class X Chapter 1 – Real Numbers Maths

Exercise 1.4
Question 1:
Without actually performing the long division, state whether the
following rational numbers will have a terminating decimal expansion
or a non-terminating repeating decimal expansion:

Answer:

(i)

The denominator is of the form 5m.

Hence, the decimal expansion of is terminating.

(ii)

The denominator is of the form 2m.

Hence, the decimal expansion of is terminating.

(iii)

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455 = 5 × 7 × 13

Since the denominator is not in the form 2m × 5n, and it also contains
7 and 13 as its factors, its decimal expansion will be non-terminating
repeating.

(iv)

1600 = 26 × 52
The denominator is of the form 2m × 5n.

Hence, the decimal expansion of is terminating.

(v)

Since the denominator is not in the form 2m × 5n, and it has 7 as its

factor, the decimal expansion of is non-terminating repeating.

(vi)

The denominator is of the form 2m × 5n.

Hence, the decimal expansion of is terminating.

(vii)

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Since the denominator is not of the form 2m × 5n, and it also has 7 as

its factor, the decimal expansion of is non-terminating

repeating.

(viii)

The denominator is of the form 5n.

Hence, the decimal expansion of is terminating.

(ix)

The denominator is of the form 2m × 5n.

Hence, the decimal expansion of is terminating.

(x)

Since the denominator is not of the form 2m × 5n, and it also has 3 as

its factors, the decimal expansion of is non-terminating repeating.

Question 2:

Write down the decimal expansions of those rational numbers in

Question 1 above which have terminating decimal expansions.

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Class X Chapter 1 – Real Numbers Maths

Answer:

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Class X Chapter 1 – Real Numbers Maths

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Class X Chapter 1 – Real Numbers Maths

(viii)

Question 3:

The following real numbers have decimal expansions as given below.

In each case, decide whether they are rational or not. If they are

rational, and of the form , what can you say about the prime factor
of q?

(i) 43.123456789 (ii) 0.120120012000120000… (iii)

Answer:
(i) 43.123456789

Since this number has a terminating decimal expansion, it is a rational

number of the form and q is of the form

i.e., the prime factors of q will be either 2 or 5 or both.
(ii) 0.120120012000120000 …

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The decimal expansion is neither terminating nor recurring. Therefore,

the given number is an irrational number.

(iii)
Since the decimal expansion is non-terminating recurring, the given

number is a rational number of the form and q is not of the form

i.e., the prime factors of q will also have a factor other than 2 or
5.

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