Class 9 Exam  >  Class 9 Notes  >  Mathematics (Maths) Class 9  >  Short Notes: Number System

Number System Class 9 Notes Maths Chapter 1

Introduction

  • The numbers of the form p/q, where ‘p’ and ‘q’ are integers and q ≠ 0, are called rational numbers.
  • A rational number p/q is said to be in the simplest form if ‘p’ and ‘q’ (q ≠ 0) are integers having no common factor other than 1.
  • Every rational number can be expressed as a decimal. If the decimal expression of p/q terminates, then it is called a terminating decimal.Number System Class 9 Notes Maths Chapter 1
  • A rational number p/q (in simplest form) is a terminating decimal only when prime factors of ‘q’ are 2 or 5 or both.
  • A decimal in which a digit or a group of digits repeats periodically is called a recurring decimal.
  • The decimal expression of a rational number is either terminating or non-terminating recurring.
  • An irrational number is a number that cannot be expressed as the ratio of two integers.
  • The decimal expression of an irrational number is non-terminating and non-recurring.
  • If ‘r’ is rational and ‘s’ is irrational, then for r ≠ 0, all r + s, r – s, r * s and r/s  are irrational.
  • The collection of all rational and irrational numbers is called real numbers.
  • The process of converting the irrational denominator of a number by multiplying its numerator and denominator by a suitable number is called rationalization.

Question for Short Notes: Number System
Try yourself:The decimal expansion of an irrational number may be:
View Solution

1. Natural Numbers


  • Counting numbers like 1, 2, 3, 4, … are called natural numbers. 
  • We denote the collection of natural numbers by ‘N’.

2. Whole Numbers


  • All natural numbers together with zero are called whole numbers. 
  • We denote the group of whole numbers by the symbol ‘W’.

3. Integers


  • The negative numbers, the positive numbers, and zero together form the set of integers.
  •  The set (collection) of all integers is denoted by the symbol ‘Z’.

Note: The symbol Z is taken from the German word “Zählen”, which means “to count”.

4. Rational Numbers

  • A number that can be represented in the form of p/q is called a rational number. 
  • Rational numbers include natural numbers, integers, and all the (negative and positive) fractions. 
  • The collection of rational numbers is denoted by the symbol ‘Q’.
    Example: 1/2, 4/5, 26/8, etc.

Note: The symbol Q comes from the word ‘quotient’ and the word ‘rational’ comes from ‘ratio’.

Rational Numbers Between Two Given Rational Numbers:


  • There are an infinite number of rational numbers between any two given rational numbers.
  • Important Results
    (i) Let ‘x’ and ‘y’ be two rational numbers.
    A rational number lying between ‘x’ and ‘y’ = (x + y)/2.
    (ii) Let ‘x’ and ‘y’ be two rational numbers such that x < y.
    To find ‘n’ rational numbers between ‘x’ and ‘y’, letNumber System Class 9 Notes Maths Chapter 1 

    Then ‘n’ rational numbers between ‘x’ and ‘y’ are: (x + d); (x + 2d), (x + 3d), … (x + nd)
    This is known as the method of finding rational numbers in one step.

Question for Short Notes: Number System
Try yourself:In between two rational number there is/are:
View Solution

5. Irrational Numbers

  • A number that cannot be written in the form of p/q, (where p and q are integers and q ≠ 0), is called an irrational number. We denote an irrational number by ‘S’.
  • Examples: √2, √3, √5, √6, √11, π, 0.10110111011110...., etc., are not rational numbers, so they are irrational numbers.

Note: 
(i) An irrational number can be written as a decimal, but not as a fraction. An irrational number has endless non-repeating digits to the right of the decimal point i.e., an irrational number is an infinite decimal.
(ii) Irrational numbers are rarely used in daily life, but they do exist on the number line. In fact, on the number line, between 0 and 1, there are an infinite number of irrational numbers.

Properties of Irrational Numbers

  • These satisfy the commutative, associative and distributive laws for addition and multiplication.
  • The sum of the two irrationals need not be irrational.
    Example: (2 + √3) + (4 – √3) = 6
  • The difference of the two irrationals need not be irrational.
    Example: (5 + √2) – (3 + √2) = 2
  • The product of the two irrationals need not be irrational.
    Example: √3 x √3 = 3
  • The quotient of the two irrationals need not be irrational.
    Example: (2√3)/√3 = 2
  • The sum of rational and irrational is irrational.
  • The difference of a rational number and an irrational number is irrational.
  • The product of a rational and an irrational number is irrational.
  • The quotient of a rational and an irrational number is irrational.

Number System Class 9 Notes Maths Chapter 1

Special Properties of Rational and Irrational Numbers

  • Every rational number can either be a terminating decimal or a non-terminating repeating (recurring) decimal.
  • Every terminating decimal is a rational number.
  • Every repeating decimal is a rational number.
  • An irrational number can neither be a terminating decimal nor a repeating decimal.
  • The square root of any prime number is an irrational number.
  • If ‘m’ is a positive integer but not a perfect square, then √m is an irrational number. i.e. √1 = 1, rational number;
    √2, irrational number
    √3, irrational number
    √4 = 2, rational number
    √5, irrational number
    √6, √7, √8 irrational number
    √9 = 3 rational number

  • If ‘m’ is a positive integer but not a perfect cube, then ∛m is an irrational number. So ∛2, ∛3, ∛4, ∛5, ∛6, ∛7,.. are irrational number
     But ∛1 = 1, rational number
     ∛8 = 2, rational number
     ∛27 = 3 , rational number
     ∛64 = 4 , rational number

Question for Short Notes: Number System
Try yourself:The product of a rational and an irrational number is (if the rational number is non-zero):
View Solution

6. Real Numbers


  • The collection of all rational numbers and irrational numbers together make up a collection of real numbers.
  • The collection of real numbers is denoted by ‘R’. 
  • The square of a real number is always non-negative(≥0).

Remember
(i) Every real number is either rational or irrational.
(ii) Between any two real numbers there exists an infinite number of real numbers.
(iii) On the number line each point corresponds to a unique real number. On the other hand, every real number can be represented by a unique point on the number line. That is why we call a “number-line” as “real-line” also. All numbers positive and negative, integers and rational numbers, square-roots, cube-roots, π (pi) are present on a number line
(iv) 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.

Real Numbers and Their Decimal Expansions

(a) The Decimal Representation of Rational Numbers

  • All rational numbers can be expressed as decimal numbers. 
  • A rational number in the form of p/q, (q 0), on the division of ‘p’ by ‘q’, either the remainder becomes zero or never becomes zero.
    Examples:
    Number System Class 9 Notes Maths Chapter 1
    Remainder = 0
    Quotient = 0.375
    Here, the decimal expansion ends (terminates) after a finite number of steps, such numbers are called terminating.
    Number System Class 9 Notes Maths Chapter 1
    Remainder: Every time we are getting the remainder 2, which will never end.
    Quotient: 0.666…, i.e. the digit 6 is repeating every time.
    Here, the decimal expansion does not end (does not terminate) even after an infinite number of steps. Such numbers are called non-terminating.
    (iii)
    Number System Class 9 Notes Maths Chapter 1
    Remainder: We are getting the remainder as 3 and 8 alternatively.
    Quotient: The digits after the decimal point are repeating in a block of two digits, i.e. ‘2’ and ‘7’.
    In such cases, the decimal expansion go on forever and we get a repeating block of digits in the quotient. We call such numbers non-terminating repeating (or non-terminating recurring).

Remember
(i) The number of digits in the repeating block is always less than the divisor.
(ii) The decimal expansion of a rational number is either terminating or non-terminating recurring (repeating).

Question for Short Notes: Number System
Try yourself:The decimal form of 3/13 is ————–
View Solution

(b) The Decimal Representation of Irrational Numbers

  • The decimal expansion of an irrational number is non-terminating and non-recurring (non-repeating).
  • Example:
    (i) 0.10110111011110....
    (ii) √3 = 1.732050807...
    (iii) √7 = 2.645751311 ...
    (iv) π = 3.14159265358 ....
    (v) √2 = 1.41421356237 ....

Note: Generally we take π = 22/7 which is not true. In fact π is an irrational number having a value which is non-terminating and non-repeating, i.e. π = 3.1415926535897932384… whereas 22/7 is rational number whose value is non-terminating repeating, i.e. 22/7 = 3.142857142857…
Thus, π and 22/7 are same only up to two places of decimal.

Rationalization

  • When a number is such that its denominator is irrational, then we need it to be rationalized.
  • The process of converting an irrational denominator of a number to a rational number by multiplying its numerator and denominator by a suitable number is called rationalization.

Rationalizing Factor (RF)

If the product of two rational numbers is rational, then each one is called the rationalizing factor of the other.
(i) If ‘a’ and ‘b’ are integers, then
(a + √b) and (a - √b) are RF of each other.
(ii) If ‘x’ and ‘y’ are natural numbers, then
 (√x+ √y) and (√x - √y) are RF of each other.
(iii) If ‘a’ and ‘b’ are integers and ‘x’ and ‘y’ are natural numbers, then  
(a +b√x) and (a -b√x) are RF of each other.

Laws of Exponents

If a > 0 and b > 0 be real numbers, ‘m’ and ‘n’ be rational numbers, then we have

  • am x an = am+n
  • am ÷  an = am-n
  • (am)n = amn
  • am x  bn = (ab)m
  • am/bm = (a/b)m
  • a= 1 
  • a-n = 1/an

Representing Real Numbers on the Number Line (Old Syllabus)

  • We know that there is a unique real number corresponding to every point on the number line. Also, corresponding to each real number, there is a unique point on the number line. 
  • The process of successive magnification helps us to locate a real number on the number line.

Note: The process of visualization of representing a decimal expansion on the number line is known as the process of successive magnification.

The document Number System Class 9 Notes Maths Chapter 1 is a part of the Class 9 Course Mathematics (Maths) Class 9.
All you need of Class 9 at this link: Class 9
48 videos|387 docs|65 tests

Up next

FAQs on Number System Class 9 Notes Maths Chapter 1

1. What are natural numbers?
Ans. Natural numbers are a set of positive integers starting from 1 and going up to infinity. They are used to count and represent quantities.
2. What are whole numbers?
Ans. Whole numbers are a set of non-negative integers that include natural numbers along with zero. They start from 0 and go up to infinity.
3. What are integers?
Ans. Integers are a set of both positive and negative whole numbers along with zero. They include all the natural numbers, whole numbers, and their negatives.
4. What are rational numbers?
Ans. Rational numbers are numbers that can be expressed as a ratio of two integers, where the denominator is not zero. They can be written in the form of p/q, where p and q are integers and q is not equal to zero.
5. What are irrational numbers?
Ans. Irrational numbers are numbers that cannot be expressed as a ratio of two integers. They cannot be written in the form of p/q, where p and q are integers and q is not equal to zero. Examples of irrational numbers include pi (π) and the square root of 2 (√2).
48 videos|387 docs|65 tests
Download as PDF

Up next

Explore Courses for Class 9 exam
Signup for Free!
Signup to see your scores go up within 7 days! Learn & Practice with 1000+ FREE Notes, Videos & Tests.
10M+ students study on EduRev
Download the FREE EduRev App
Track your progress, build streaks, highlight & save important lessons and more!
Related Searches

MCQs

,

study material

,

Viva Questions

,

mock tests for examination

,

past year papers

,

Semester Notes

,

Summary

,

practice quizzes

,

Important questions

,

Number System Class 9 Notes Maths Chapter 1

,

Free

,

Exam

,

ppt

,

Sample Paper

,

Extra Questions

,

Previous Year Questions with Solutions

,

Objective type Questions

,

video lectures

,

shortcuts and tricks

,

Number System Class 9 Notes Maths Chapter 1

,

Number System Class 9 Notes Maths Chapter 1

,

pdf

;