Class IX Unit-1 Number System

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What is Number

A number is a mathematical object used to count, measure, and label. The original examples are the natural numbers 1, 2, 3, and so forth. A notational symbol that represents a number is called a numeral. In addition to their use in counting and measuring, numerals are often used for labels (as with telephone numbers), for ordering (as with serial numbers), and for codes (as with ISBNs). In common usage, number may refer to a symbol, a word, or a mathematical abstraction.

In mathematics, the notion of number has been extended over the centuries to include 0, negative numbers, rational numbers such as 1/2 and -1/2, real numbers such as &radic2 , complex numbers, which extend the real numbers by including &radic(-1), and sometimes additional objects. Calculations with numbers are done with arithmetical operations, the most familiar being addition, subtraction, multiplication, division, and exponentiation. Their study or usage is called arithmetic. The same term may also refer to number theory, the study of the properties of the natural numbers.

Natural and Whole Number

What is Natural Numbers?
Set of counting numbers is called the Natural Numbers
N = {1,2,3,4,5,...}
What is whole number?
Set of Natural numbers plus Zero is called the Whole Numbers
W= {0,1,2,3,4,5,....}
Note: 
So all natural Number are whole number but all whole numbers are not natural numbers

Examples:
2 is Natural Number
-2 is not a Natural number
0 is a whole number 

Integers

What are Integers Numbers
Integers is the set of all the whole number plus the negative of Natural Numbers
Z={..,-7,-6,-5,-4,-3,-2,-1,0,1,2,3,4,5,6,..}
Note
1) So integers contains all the whole number plus negative of all the natural numbers
2)the natural numbers without zero are commonly referred to as positive integers
3)The negative of a positive integer is defined as a number that produces 0 when it is added to the corresponding positive integer 
4)natural numbers with zero are referred to as non-negative integers
5) The natural numbers form a subset of the integers.

Rational and Irrational Numbers

Rational Number

: A number is called rational if it can be expressed in the form p/q where p and q are integers ( q> 0).
Example : 1/2, 4/3 ,5/7 ,1 etc.
Important Points to Note

• every integers, natural and whole number is a rational number as they can be expressed in termsof p/q
• There are infinite rational number between two rational number
• They either have termination decimal expression or repeating non terminating decimal expression.SO if a number whose decimal expansion is terminating or non-terminating recurring then it is rational
• The sum, difference and the product of two rational numbers is always a rational number. The quotient of a division of one rational number by a non-zero rational number is a rational number. Rational numbers satisfy the closure property under addition, subtraction, multiplication and division.

  Irrational Number

  : A number is called rational if it cannot be expressed in the form p/q where p and q are integers ( q> 0).
  Example : √3,√2,√5,p etc
  Important Points to Note
  
  • Pythagoras Theorem: In a right-angled triangle, the square of the hypotenuse is equal to the sum of the squares of the other two sides. Using this theorem we can represent the irrational numbers on the number line.
  • They have non terminating and non repeating decimal expression. If a number is non terminating and non repeating decimal expression,then it is irrational number
  • The sum, difference, multiplication and division of irrational numbers are not always irrational. Irrational numbers do not satisfy the closure property under addition, subtraction, multiplication and division
  
  
  Example
  Write the following in decimal form and say what kind of decimal expansion each has:
  (i) 15/100
  
  (ii) 1/9
  
  (iii)  2/11
  
  (iv) 3/13
  
  Answer
  

  i)	15/100	0.15 (Terminating)
  ii)	1/9	0.111111... (Non terminating repeating)
  iii)	2/11	.18181818....(Non terminating repeating)
  iv)	3/13	0.230769230769... = 0.230769 (Non terminating repeating)

  
  Example
  Express the following in the form p/q where p and q are integers and q ≠ 0.
  
  Answer
  
  Let x = 0.777...
  10x = 7.777...
  10x = 7 + x
  9x = 7
  x = 7/9

  Real Numbers:

  
  
  • All rational and all irrational number makes the collection of real number. It is denoted by the letter R
  • We can represent real numbers on the number line. The square root of any positive real number exists and that also can be represented on number line
  • The sum or difference of a rational number and an irrational number is an irrational number.
  • The product or division of a rational number with an irrational number is an irrational number.
  • This process of visualization of representing a decimal expansion on the number line is known as the process of successive magnification
  
  
  Real numbers satisfy the commutative, associative and distributive laws. These can be stated as :
  Commutative Law of Addition:
  a+b= b+a
  Commutative Law of Multiplication:
  a X b=b X a
  Associative Law of Addition:
  a + (b+c)=(a+b) +c 
  Associative Law of Multiplication:
  a X (b X c)=(a X b) X c 
  Distributive Law:
  a X (b + c)=(a X b) + (a X c) 
  or
  (a + b) X c=(a X c) + (b X c)