Mathematics NCERT Grade 9, Chapter 1: Number Systems- Basics of number line and representation on number line is already discussed in earlier classes. Rational numbers and Irrational numbers are defined in this chapter
•  A number r is called a rational number, if it can be written in the form p/q, where p and q are integers and q $\ne$ 0.
Irrational numbers are also explained in section 1.2.
• A number s is called an irrational number, if it cannot be written in the form of p/q, where p and q are integers and q$\ne$0.
One can learn the representation of the irrational number on the number line in section 1.2.
A short exercise 1.2 is given which also has a small activity on square root spiral. Moving forward students will learn about real numbers and their decimal expansions. ​
• The decimal expansion of a rational number is either terminating or non-terminating recurring.
• The decimal expansion of an irrational number is non-terminating and non-recurring.
• Real numbers are made up of the collection of rational numbers and irrational numbers.
In the next section, the topic of representing real numbers on the number line is explained through successive magnification.
• Real number is represented by a unique point on the number line
• Point on the number line represents one and only one real number
Exercise 1.4 contains only two questions. After that, operations on real numbers is discussed. In this section rationalization of the denominator is explained. Section 1.6 is about the laws of exponents for real numbers. Some important rules are discussed in this section. Students must remember these points to solve the problems on the same topic. Students can attempt the exercise 1.6 after going through the examples.
Later in section 1.7 summary of the chapter Number Systems is given.

#### Question 1:

Is zero a rational number? Can you write it in the form, where p and q are integers and q ≠ 0?

#### Answer:

Yes. Zero is a rational number as it can be represented asetc.

##### Video Solution for number systems (Page: 5 , Q.No.: 1)

NCERT Solution for Class 9 math - number systems 5 , Question 1

#### Question 2:

Find six rational numbers between 3 and 4.

#### Answer:

There are infinite rational numbers in between 3 and 4.

3 and 4 can be represented asrespectively.

Therefore, rational numbers between 3 and 4 are

##### Video Solution for number systems (Page: 5 , Q.No.: 2)

NCERT Solution for Class 9 math - number systems 5 , Question 2

#### Question 3:

Find five rational numbers between.

#### Answer:

There are infinite rational numbers between.

Therefore, rational numbers betweenare

##### Video Solution for number systems (Page: 5 , Q.No.: 3)

NCERT Solution for Class 9 math - number systems 5 , Question 3

#### Question 4:

State whether the following statements are true or false. Give reasons for your answers.

(i) Every natural number is a whole number.

(ii) Every integer is a whole number.

(iii) Every rational number is a whole number.

#### Answer:

(i) True; since the collection of whole numbers contains all natural numbers.

(ii) False; as integers may be negative but whole numbers are positive. For example: −3 is an integer but not a whole number.

(iii) False; as rational numbers may be fractional but whole numbers may not be. For example: is a rational number but not a whole number.

##### Video Solution for number systems (Page: 5 , Q.No.: 4)

NCERT Solution for Class 9 math - number systems 5 , Question 4

#### Question 1:

State whether the following statements are true or false. Justify your answers.

(i) Every irrational number is a real number.

(ii) Every point on the number line is of the form, where m is a natural number.

(iii) Every real number is an irrational number.

#### Answer:

(i) True; since the collection of real numbers is made up of rational and irrational numbers.

(ii) False; as negative numbers cannot be expressed as the square root of any other number.

(iii) False; as real numbers include both rational and irrational numbers. Therefore, every real number cannot be an irrational number.

##### Video Solution for number systems (Page: 8 , Q.No.: 1)

NCERT Solution for Class 9 math - number systems 8 , Question 1

#### Question 2:

Are the square roots of all positive integers irrational? If not, give an example of the square root of a number that is a rational number.

#### Answer:

If numbers such asare considered,

Then here, 2 and 3 are rational numbers. Thus, the square roots of all positive integers are not irrational.

##### Video Solution for number systems (Page: 8 , Q.No.: 2)

NCERT Solution for Class 9 math - number systems 8 , Question 2

#### Question 3:

Show howcan be represented on the number line.

#### Answer:

We know that,

And,

Mark a point ‘A’ representing 2 on number line. Now, construct AB of unit length perpendicular to OA. Then, taking O as centre and OB as radius, draw

an arc intersecting number line at C.

C is representing.

##### Video Solution for number systems (Page: 8 , Q.No.: 3)

NCERT Solution for Class 9 math - number systems 8 , Question 3

#### Question 1:

Write the following in decimal form and say what kind of decimal expansion each has:

(i) (ii) (iii)

(iv) (v) (vi)

#### Answer:

(i)

Terminating

(ii)

Non-terminating repeating

(iii)

Terminating

(iv)

Non-terminating repeating

(v)

Non-terminating repeating

(vi)

Terminating

##### Video Solution for number systems (Page: 14 , Q.No.: 1)

NCERT Solution for Class 9 math - number systems 14 , Question 1

#### Question 2:

You know that. Can you predict what the decimal expansion of are, without actually doing the long division? If so, how?

[Hint: Study the remainders while finding the value of carefully.]

#### Answer:

Yes. It can be done as follows.

##### Video Solution for number systems (Page: 14 , Q.No.: 2)

NCERT Solution for Class 9 math - number systems 14 , Question 2

#### Question 3:

Express the following in the form, where p and q are integers and q ≠ 0.

(i) (ii) (iii)

#### Answer:

(i)

Let x = 0.666…

10x = 6.666…

10x = 6 + x

9x = 6

(ii)

Let x = 0.777…

10x = 7.777…

10x = 7 + x

(iii)

Let x = 0.001001…

1000x = 1.001001…

1000x = 1 + x

999x = 1

##### Video Solution for number systems (Page: 14 , Q.No.: 3)

NCERT Solution for Class 9 math - number systems 14 , Question 3

#### Question 4:

Express 0.99999…in the form. Are you surprised by your answer? With your teacher and classmates discuss why the answer makes sense.

#### Answer:

Let x = 0.9999…

10x = 9.9999…

10x = 9 + x

9x = 9

x = 1

##### Video Solution for number systems (Page: 14 , Q.No.: 4)

NCERT Solution for Class 9 math - number systems 14 , Question 4

#### Question 5:

What can the maximum number of digits be in the repeating block of digits in the decimal expansion of? Perform the division to check your answer.

#### Answer:

It can be observed that,

There are 16 digits in the repeating block of the decimal expansion of.

##### Video Solution for number systems (Page: 14 , Q.No.: 5)

NCERT Solution for Class 9 math - number systems 14 , Question 5

#### Question 6:

Look at several examples of rational numbers in the form (q ≠ 0), where p and q are integers with no common factors other than 1 and having terminating decimal representations (expansions). Can you guess what property q must satisfy?

#### Answer:

Terminating decimal expansion will occur when denominator q of rational number is either of 2, 4, 5, 8, 10, and so on…

It can be observed that terminating decimal may be obtained in the situation where prime factorisation of the denominator of the given fractions has the power of 2 only or 5 only or both.

##### Video Solution for number systems (Page: 14 , Q.No.: 6)

NCERT Solution for Class 9 math - number systems 14 , Question 6

#### Question 7:

Write three numbers whose decimal expansions are non-terminating non-recurring.

#### Answer:

3 numbers whose decimal expansions are non-terminating non-recurring are as follows.

0.505005000500005000005…

0.7207200720007200007200000…

0.080080008000080000080000008…

##### Video Solution for number systems (Page: 14 , Q.No.: 7)

NCERT Solution for Class 9 math - number systems 14 , Question 7

#### Question 8:

Find three different irrational numbers between the rational numbers and.

#### Answer:

3 irrational numbers are as follows.

0.73073007300073000073…

0.75075007500075000075…

0.79079007900079000079…

##### Video Solution for number systems (Page: 14 , Q.No.: 8)

NCERT Solution for Class 9 math - number systems 14 , Question 8

#### Question 9:

Classify the following numbers as rational or irrational:

(i) (ii) (iii) 0.3796

(iv) 7.478478 (v) 1.101001000100001…

#### Answer:

(i)

As the decimal expansion of this number is non-terminating non-recurring, therefore, it is an irrational number.

(ii)

It is a rational number as it can be represented in form.

(iii) 0.3796

As the decimal expansion of this number is terminating, therefore, it is a rational number.

(iv) 7.478478 …

As the decimal expansion of this number is non-terminating recurring, therefore, it is a rational number.

(v) 1.10100100010000 …

As the decimal expansion of this number is non-terminating non-repeating, therefore, it is an irrational number.

##### Video Solution for number systems (Page: 14 , Q.No.: 9)

NCERT Solution for Class 9 math - number systems 14 , Question 9

#### Question 1:

Visualise 3.765 on the number line using successive magnification.

#### Answer:

3.765 can be visualised as in the following steps.

##### Video Solution for number systems (Page: 18 , Q.No.: 1)

NCERT Solution for Class 9 math - number systems 18 , Question 1

#### Question 2:

Visualise on the number line, up to 4 decimal places.

#### Answer:

= 4.2626…

4.2626 can be visualised as in the following steps.

##### Video Solution for number systems (Page: 18 , Q.No.: 2)

NCERT Solution for Class 9 math - number systems 18 , Question 2

#### Question 1:

Classify the following numbers as rational or irrational:

(i) (ii) (iii)

(iv) (v) 2π

#### Answer:

(i) = 2 − 2.2360679…

= − 0.2360679…

As the decimal expansion of this expression is non-terminating non-recurring, therefore, it is an irrational number.

(ii)

As it can be represented in form, therefore, it is a rational number.

(iii)

As it can be represented in form, therefore, it is a rational number.

(iv)

As the decimal expansion of this expression is non-terminating non-recurring, therefore, it is an irrational number.

(v) 2π = 2(3.1415 …)

= 6.2830 …

As the decimal expansion of this expression is non-terminating non-recurring, therefore, it is an irrational number.

##### Video Solution for number systems (Page: 24 , Q.No.: 1)

NCERT Solution for Class 9 math - number systems 24 , Question 1

#### Question 2:

Simplify each of the following expressions:

(i) (ii)

(iii) (iv)

#### Answer:

(i)

(ii)

= 9 − 3 = 6

(iii)

(iv)

= 5 − 2 = 3

##### Video Solution for number systems (Page: 24 , Q.No.: 2)

NCERT Solution for Class 9 math - number systems 24 , Question 2

#### Question 3:

Recall, π is defined as the ratio of the circumference (say c) of a circle to its diameter (say d). That is, . This seems to contradict the fact that π is irrational. How will you resolve this contradiction?

#### Answer:

There is no contradiction. When we measure a length with scale or any other instrument, we only obtain an approximate rational value. We never obtain an exact value. For this reason, we may not realise that either c or d is irrational. Therefore, the fraction is irrational. Hence, π is irrational.

##### Video Solution for number systems (Page: 24 , Q.No.: 3)

NCERT Solution for Class 9 math - number systems 24 , Question 3

#### Question 4:

Represent on the number line.

#### Answer:

Mark a line segment OB = 9.3 on number line. Further, take BC of 1 unit. Find the mid-point D of OC and draw a semi-circle on OC while taking D as its centre. Draw a perpendicular to line OC passing through point B. Let it intersect the semi-circle at E. Taking B as centre and BE as radius, draw an arc intersecting number line at F. BF is.

##### Video Solution for number systems (Page: 24 , Q.No.: 4)

NCERT Solution for Class 9 math - number systems 24 , Question 4

#### Question 5:

Rationalise the denominators of the following:

(i) (ii)

(iii) (iv)

#### Answer:

(i)

(ii)

(iii)

(iv)

##### Video Solution for number systems (Page: 24 , Q.No.: 5)

NCERT Solution for Class 9 math - number systems 24 , Question 5

Find:

(i) (ii) (iii)

#### Answer:

(i)

(ii)

(iii)

##### Video Solution for number systems (Page: 26 , Q.No.: 1)

NCERT Solution for Class 9 math - number systems 26 , Question 1

Q2. Find:

(i) (ii) (iii)

(iv)

#### Answer:

(i)

(ii)

(iii)

(iv)

##### Video Solution for number systems (Page: 26 , Q.No.: 2)

NCERT Solution for Class 9 math - number systems 26 , Question 2

Simplify:

(i) (ii) (iii)

(iv)

#### Answer:

(i)

(ii)

(iii)

(iv)

##### Video Solution for number systems (Page: 26 , Q.No.: 3)

NCERT Solution for Class 9 math - number systems 26 , Question 3

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