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**Number System Formulas:**

**Even Numbers: **Numbers which are divisible by 2

**Example:** 2,4,6,8,10,12,14……etc

**Odd Numbers:** Numbers which are not divisible by 2

**Example:** 1,3,5,7,9,11,13,15,17……..etc

**Natural Numbers: **All counting numbers

**Example:** N ={1,2,3,4,…….. etc}

Sum of first ‘n’ natural numbers is n(n+1) / 2

Sum of squares of first ‘n’ natural numbers is n(n+1)(2n+1) / 6

Sum of cubes of first ‘n’ natural numbers is ( n(n+1) / 2)^{2}

**Whole Numbers: ** 0 and all natural numbers

**Example:** 0,1,2,3,4,5,6,…… etc.

**Prime Numbers:** Numbers divisible by one and the number itself (Or) Numbers having only two factors.

**Example:** 2,3,5,7,11,13,17,19…. etc.

(i) All prime numbers end with 1,3,7,9 except 2 and 5

(ii) 2 is the even prime number

(iii) ‘1’ is neither a prime nor a composite number. It is a unitary

(iv) There are 25 prime numbers upto 100

There are 2,3,5,7,11,13,17,19,23,29,31,37,41,43,447,53,59,61,67,71,73,79,83,97.

**Co prime Numbers:** Numbers not a having any common factors.

**Example:** 3,5,8 are co primes to each other.

**Twin prime number:** The difference between two prime numbers is ‘2’

**Example:** 3 – 5 , 5 -7 , 11 – 13, 71 – 73 etc

**Composite Numbers:** Numbers having more than two factors.

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