Following are the classifications of numbers.

### 1. Natural Numbers:

- Each of 1,2,3,4,…..,etc is a natural number.
- The smallest natural number is 1 ;whereas the largest natural number cannot be obtained.
- Consecutive natural numbers differ by 1.
- Let be any natural number , then the natural numbers that come just after are etc.

### 2. Even Natural Numbers:

A system of natural numbers ,which are divisible by 2 or are multiples of 2, is called a set of even numbers.

E= (2,4,6,8,10,12……..)

There are infinite even numbers.

### 3. Odd Natural Numbers:

A system of natural numbers ,which are not divisible by 2 ,is called a set of odd numbers.

O= (1,3,5,7,9………)

There are infinite odd numbers.

Taking together the odd and even numbers, we get natural numbers.

### 4. Whole Numbers:

- 0,1 ,2,3,4,…… etc are whole numbers.
- The smallest whole number is zero whereas the largest whole number cannot be obtained.
- Consecutive whole numbers differ by 1.
- Except zero every whole number is a natural number and because of this:
- Every even natural number is an even whole number
- Every odd natural number is an odd whole number.

### 5. Prime Numbers:

- Whole numbers greater than 1 that are divisible by unity and itself only.
- Except 2 all other prime numbers are odd. P= 2,3,5,7,11,13,………. etc.

### 6. Composite Numbers:

A composite number is a whole number (greater than 1) that is not prime.

Composite numbers C= (4,6,8,9 ……..,etc)

### 7. Integers:

- The integers consists of natural numbers , zero and negative of natural numbers. Thus , Z or I = …………………,-4,-3,-2,-1, 0 , 1,2,3,4…………….
- There are infinite integers towards positive side and infinite integers towards negative side .
- Positive integers are the natural numbers.

**Use of Integers**

The integers are used to express our day-to-day situations in mathematical terms.

- If profits are represented by positive integers then losses by negative integers.
- If heights above sea level by positive integers then depths below sea level by negative integers.
- If rise in price is represented by positive integers ,then fall in price by negative integers and so on.

### 8. Rational Numbers:

Any number which can be expressed in the form of , where a and b both are integers and , is a rational number.

- is a rational number, since 2, 5 are integers and 5 is not equal to zero.
- , etc are not rational numbers since these numbers cannot be expressed as .

So, we can say that rational numbers contain all integers and all fractions (including decimals). There are infinite number of rational numbers.

- Every integer is a rational number but the converse is not true. The same result is true for natural numbers, whole numbers, fractions, etc.

### 9. Irrational Numbers:

Then numbers which are not rational are called irrational numbers .

Each of , etc is an irrational number.

The number is neither rational nor irrational if .

### 10. Real Numbers:

Every number, which is either rational or irrational is called a real number.

- Each natural number is a real number.
- Each whole number is a real number.
- Each integer is a real number.
- Each rational number is a real number.
- Each irrational number is a real number , etc.

## Absolute Value of a Number:

The absolute value of an integer is its numerical value regardless of it’s sign.

Absolute value of

Absolute value of

Therefore if represents an integer, its absolute value is represented by and is always non-negative

### Remember:

- , when is positive or zero
- , when is negative.

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