# Number System for Class 9

Number system for class 9 which is the first chapter has been given here for students to get a reference for the same. Here you will learn about the Number System with its definition and types of numbers. Also, learn the definition of all the types along with their properties. Students who are preparing for the exam could use these materials as a source of learning and have revision at the time of the final exams. Get some example questions and practice questions to understand the number system.

## Introduction to number system class 9

The collection of numbers is called the number system. These numbers are of different types such as natural numbers, whole numbers, integers, rational numbers and irrational numbers. Let us see the table below to understand with the examples.

 Natural Numbers N 1, 2, 3, 4, 5, … Whole Numbers W 0,1, 2, 3, 4, 5… Integers Z …., -3, -2, -1, 0, 1, 2, 3, … Rational Numbers Q p/q form, where p and q are integers and q is not zero. Irrational Numbers Which can’t be represented as rational numbers

### Natural Numbers

All the numbers starting from 1 till infinity are natural numbers, such as 1,2,3,4,5,6,7,8,…….infinity. These numbers lie on the right side of the number line and are positive.

### Whole Numbers

All the numbers starting from 0 till infinity are whole numbers such as 0,1,2,3,4,5,6,7,8,9,…..infinity. These numbers lie on the right side of the number line from 0 and are positive.

### Integers

Integers are the whole numbers which can be positive, negative or zero.

Example: 2, 33, 0, -67 are integers.

### Rational Numbers

A number which can be represented in the form of p/q is called a rational number. For example, 1/2, 4/5, 26/8, etc.

### Irrational Numbers

A number is called an irrational number if it can’t be represented in the form of ratio.

Example: √3, √5, √11, etc.

### Real Numbers

The collection of all rational and irrational numbers is called real numbers. Real numbers are denoted by R.

Every real number is a unique point on the number line and also every point on the number line represents a unique real number.

## Difference between Terminating and Recurring Decimals

 Terminating Decimals Repeating Decimals If the decimal expression of a/b terminates. i.e. comes to an end, then the decimal so obtained is called Terminating decimals. A decimal in which a digit or a set of digits repeats repeatedly periodically is called a repeating decimal. Example: ¼ =0.25 Example: ⅔ = 0.666…

### Some Special Characteristics of Rational Numbers

• Every Rational number is expressible either as a terminating decimal or as a repeating decimal.
• Every terminating decimal is a rational number.
• Every repeating decimal is a rational number.

Irrational Numbers

• The non-terminating, non-repeating decimals are irrational numbers.

Example: 0.0100100001001…

• Similarly, if m is a positive number which is not a perfect square, then √m is irrational.

Example: √3

• If m is a positive integer which is not a perfect cube, then 3√m is irrational.

Example: 3√2

### Properties of Irrational Numbers

• These satisfy the commutative, associative and distributive laws for addition and multiplication.
• Sum of two irrationals need not be irrational.

Example: (2 + √3) + (4 – √3) = 6

• Difference of two irrationals need not be irrational.

Example: (5 + √2) – (3 + √2) = 2

• Product of two irrationals need not be irrational.

Example: √3 x √3 = 3

• The quotient of two irrationals need not be irrational.

2√3/√3 = 2

• Sum of rational and irrational is irrational.
• The difference of rational and irrational number is irrational.
• Product of rational and irrational is irrational.
• Quotient of rational and irrational is irrational.

## Real Numbers

A number whose square is non-negative is called a real number.

• Real numbers follow Closure property, associative law, commutative law, the existence of a multiplicative identity, existence of multiplicative inverse, Distributive laws of multiplication over Addition for Multiplication.

### Rationalisation

If we have an irrational number, then the process of converting the denominator to a rational number by multiplying the numerator and denominator by a suitable number is called rationalisation.

Example:

3/√2 = (3/ √2) x (√2/√2) = 3 √2/2

Let a>0 be a real number, and let p and q be rational numbers, then we have:

i) (ap).aq = a(p+q)

ii) (ap)q = apq

iii)ap/aq = a(p-q)

iv) ap x bp = (ab)p

Example: Simplify (36)½

Solution: (62)½ = 6(2 x ½) = 61 = 6

### Class 9 Number system Extra questions

Q1) The simplest form of $1.\tilde{6}$ is?

Q2) An irrational number between √2and √3 is?

Q3) Give an example of two irrational numbers whose sum as well as the product is rational.

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