Class 9 NUMBER SYSTEMS FOR CBSE NCERT

Introduction

### Introduction

In your earlier classes, we've learnt about the number line and how to represent various types of numbers on it (see Fig. 1.1).

Just imagine you start from zero and go on walking along this number line in the positive direction.
As far as your eyes can see, there are numbers, numbers and numbers

Now suppose you start walking along the number line, and collecting some of the numbers. Get a bag ready to store them.

You might begin with picking up only natural numbers like 1, 2, 3, and so on. You know that this list goes on for ever.
So, now your bag contains infinitely many natural numbers! Recall that we denote this collection by the symbol N.

Now turn and walk all the way back, pick up zero and put it into the bag. You now have the collection of whole numbers which is denoted by the symbol W.

Now, stretching in front of you are many, many negative integers. Put all the negative integers into your bag.

Now, Recall that it is the collection of all integers, and it is denoted by the symbol Z.

Are there some numbers still left on the line? Of course! There are numbers like color{orange}(1/2,3/4,) or even color{orange}((-2005)/(2006)) If you put all such numbers also into the bag, it will now be the collection of rational numbers.

The collection of rational numbers is denoted by Q. ‘Rational’ comes from the word ‘ratio’, and Q comes from the word ‘quotient’.

You may recall the definition of rational numbers:

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 ne0

"Notice" that all the numbers now in the bag can be written in the form p/q where p and q are integers and q ne 0
For example, –25 can be written as (-25)/1 here p = -25 and q = 1.

Therefore, the rational numbers also include the natural numbers, whole numbers and integers.
You also know that the rational numbers do not have a unique representation in the form p/q where p and q are integers and q ne 0 For example 2 = 2/4 = 10/20=25/50 = 47/94 and so on.

These are equivalent rational numbers (or fractions). However, when we say that p/q is rational number, or when we represent p/q on the number line, we assume that q ne 0 and that p and q have no common factors other than 1 (that is, p and q are co-prime).

So, on the number line, among the infinitely many fractions equivalent to 1/2 we will choose 1/2 to represent all of them.
Q 3240245113

Are the following statements true or false? Give reasons for your answers.
(i) Every whole number is a natural number.
(ii) Every integer is a rational number.
(iii) Every rational number is an integer.
Class 9 Chapter 1 Example 1
Solution:

(i) False, because zero is a whole number but not a natural number.

(ii) True, because every integer m can be expressed in the form m/1 and so it is a rational number.
(iii) False, because 3/5 is not an integer.
Q 3260245115

Find five rational numbers between 1 and 2.
We can approach this problem in at least two ways.
Class 9 Chapter 1 Example 2
Solution:

Recall that to find a rational number between r and s, you can add r and and divide the sum by 2, that is (r+s)/2 lies between r and s. So, 3/2 is a number between 1 and 2. You can proceed in this manner to find four more rational numbers between 1 and 2. These four numbers are 5/2, (11)/8, (13)/8 and 7/4.

The other option is to find all the five rational numbers in one step. Since we want five numbers, we write 1 and 2 as rational numbers with denominator 5 + 1, i.e., 1 = 6/6 and 2 = (12)/6 Then you can check that 7/5 , 8/6,9/6,(10)/6 and (11)/6 are all rational numbers between 1 and 2. So, the five numbers are 7/6,4/3,3/2,5/3 and 11/6.

### Remark

Notice that in Example 2, you were asked to find five rational numbers between 1 and 2. But, you must have realised that in fact there are infinitely many rational numbers between 1 and 2. In general, there are infinitely many rational numbers between any two given rational numbers.

So we are left with the following questions:

1. What are the numbers, that are left on the number line, called?

2. How do we recognise them? That is, how do we distinguish them from the rationals (rational numbers)?

These questions will be answered in the next section.