####
Types of number:

1.

__Natural Numbers:__ 1,2,3,… so on

2.

__Whole numbers:__ 0,1,2,3… so on

3.

__Integers:__ it can be negative or positive numbers. Negative numbers are -2, -1 and Positive numbers are 1,2,3..so on and ‘0’ is non-positive and non-negative number.

4.

__Prime numbers:__ the number which is divisible by itself or 1. Eg, 2,3,5..so on. ‘2’ is the smallest prime number.

5.

__Perfect Number:__ If the sum of the divisors of a number N excluding N itself is equal to N, then N is called perfect number. For example:

6= 1+2+3 (divisors of 6)= 6 ->
hence it is perfect number

28= 1+2+4+7+14=28

6. __Composite number:__ numbers other than
prime number. Eg, 4, 6, 8, 10.. so on

7. __Co-prime numbers;__ the combination of
numbers whose HCF is 1. Eg, (2,3), (4,5), (7,9) etc

__Note:__ ‘1’ is neither prime nor composite number.

####
-Important Formulae*:*

- The sum of
*n* natural numbers= **n(n+1)/2**
- The sum of
*even* natural numbers= **n(n+1)**
- The sum of
*odd* natural numbers= **n**^{2}
- The sum of
*square of n* natural numbers= **n(n+1)(2n+1)/6**
- The sum of
*cube of n* natural numbers= **[n(n+1)/2]**^{2}

####
-__Important Points
to remember:__

- The
*smallest*
1 digit prime number is **2**
- The
*smallest*
2 digit prime number is **11**
- The
*smallest*
3 digit prime number is **101**
- The
*smallest*
4 digit prime number is **1009**

- The
*largest*
1 digit prime number is **7**
- The
*largest*
1 digit prime number is **97**
- The
*largest*
1 digit prime number is **997**
- The
*largest*
1 digit prime number is **9973**

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