- Natural and Whole Number
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- Integers
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- Rational and Irrational Numbers
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- Real Numbers
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- Laws of exponents
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- What is Number Line
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- successive Magnification

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.

Set of counting numbers is called the Natural Numbers

$N = {1,2,3,4,5,...}$

Set of Natural numbers plus Zero is called the Whole Numbers

$W= {0,1,2,3,4,5,....}$

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

2 is Natural Number

-2 is not a Natural number

0 is a whole number

Integers is the set of all the whole number plus the negative of Natural Numbers

$Z={..,-7,-6,-5,-4,-3,-2,-1,0,1,2,3,4,5,6,..}$

- So integers contains all the whole number plus negative of all the natural numbers
- the natural numbers without zero are commonly referred to as positive integers
- The negative of a positive integer is defined as a number that produces 0 when it is added to the corresponding positive integer
- natural numbers with zero are referred to as non-negative integers
- The natural numbers form a subset of the integers.

Example : $\frac {1}{2}, \frac {4}{3},\frac {5}{7} ,1$ etc.

- 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.

Example : $\sqrt {3},\sqrt {2},\sqrt {5}$ etc

- 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

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

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) |

Express the following in the form

Let $x = 0.777...$

$10x = 7.777...$

$10x =.7+ x$

$9x = 7$

$x = \frac {7}{9}$

- 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:

$a \times b=b \times a$

Associative Law of Addition:

$ a + (b+c)=(a+b) +c $

Associative Law of Multiplication:

$ a \times (b \times c)=(a \times b) \times c $

Distributive Law:

$a \times (b + c)=(a \times b) + (a \times c)$

or

$(a + b) \times c=(a \times c) + (b \times c)$

Commutative Law of Addition:

$a+b= b+a$

Commutative Law of Multiplication:

$a \times b=b \times a$

Associative Law of Addition:

$ a + (b+c)=(a+b) +c $

Associative Law of Multiplication:

$ a \times (b \times c)=(a \times b) \times c $

Distributive Law:

$a \times (b + c)=(a \times b) + (a \times c)$

or

$(a + b) \times c=(a \times c) + (b \times c)$

$ a^{\frac {m}{n}} =(\sqrt[n]{a})^m =\sqrt[n]{a^m}$

B) Let a > 0 be a real number and p and q be rational numbers. Then, we have

- $a^p.a^q=a^{p+q}$
- $ \frac {a^p}{a^q} =a^{p-q}$
- $(a^p)^q=a^{pq}$
- $a^p.b^p=(ab)^p$

- $ \sqrt {p} \sqrt {q}= \sqrt {pq}$
- $ \sqrt {\frac {p}{q}} =\frac {\sqrt {p}}{\sqrt {q}}$
- $(\sqrt {p} + \sqrt {q})(\sqrt {p} - \sqrt {q}) =p -q$
- $(p+ \sqrt {q})(p - \sqrt {q}) = p^2 - q$
- $(\sqrt {p} + \sqrt {q})^2 = p + 2 \sqrt {pq} + q$

- A number line is a line which represent all the number. A number line is a picture of a straight line on which every point is assumed to correspond to a real number and every real number to a point
- We most shows the integers as specially-marked points evenly spaced on the line. but the line includes all real numbers, continuing forever in each direction, and also numbers not marked that are between the integers.
- It is often used as an aid in teaching simple addition and subtraction, especially involving negative numbers.
- The number on the right side are greater than number on the left side

- Natural Number,whole Number and integers can be easily located on the number line as we picture as per them
- Now Real number can be either decimal expression or number explained in point 1. It is easy to located the latter one. For decimal expression, we need to use the process of successive Magnification
- Number like $ \sqrt {3}$ can be represent on number like using pythogorus theorem

2) Are integers rational Number ? True/false

3) Are negative number rational Numbers? True/false

4) There are infinite real numbers between 1.2 and 1.3 . True/False

Check your answers

$(5 + \sqrt {2}) (5 - \sqrt {2})$

As

$(a + \sqrt {b}) (a - \sqrt {b})=a^2 -b$

So

$(5 + \sqrt {2}) (5 - \sqrt {2})$

$=25-2= 23$

$( \sqrt {9} + \sqrt {2}) ( \sqrt {9} - \sqrt {2})$

As

$(\sqrt {a} + \sqrt {b}) (\sqrt {a} - \sqrt {b})=a -b$

So

$( \sqrt {9} + \sqrt {2}) ( \sqrt {9} - \sqrt {2})$

=9-2= 7

Given below are the links of some of the reference books for class 9 Math.

- Mathematics for Class 9 by R D Sharma One of the best book for studying class 9 level mathematics. It has lot of problems to be solved.
- Secondary School Mathematics for Class 9 by R S Aggarwal This is also as good as R.D. Sharma. Either this or the book by R.D. Sharma will do. I find book R.S. Aggarwal little bit more challenging than the one by R.D. Sharma.
- Pearson IIT Foundation Series - Maths - Class 9 Buy this book if you want to challenge yourself further and want to prepare for JEE foundation.
- Pearson IIT Foundation Physics, Chemistry & Maths combo for Class 9 Only buy if you are prepared to study extra topics and want to take your studies a step further. You might need help to understand topics in these books.

You can use above books for extra knowledge and practicing different questions.

Class 9 Maths Class 9 Science

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