# Number System Formulas @Lifebix

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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.