In this section, we will talk about "prime numbers" and "prime factors". The definition of "prime number" is that it is a number that has no other factors except itself and 1. The number "1" is not regarded as a prime number.


Is "2" a prime number?....well ask yourself is it divisible by itself? Answer yes it is, is it divisible by 1, Answer yes it is. this makes "2" a prime number.


Q. Which of the following are prime numbers? - 21, 22, 23, 24, 25, 26, 27, 28, 29....?


A. All of these numbers are divisible by 1, and of course divisible by themselves. If we can stop at this point with any of the numbers we can conclude that they are prime, but we need to examine each number in turn to see if it will divide by any other number apart from one and itself.


Let's draw a table:


21

Divisible by 3, 7 as well as 1 and itself therefore 21 is not prime

22

Divisible by 11 as well as 1 and itself, therefore 22 is not prime

23

Divisible only by 1 and itself, so 23 is a prime number

24

Divisible by 2, 4, 6 and 12 as well as 1 and itself therefore 24 is clearly not a prime number

25

Divisible by 5 as well as 1 and itself therefore 25 cannot be a prime number

26

Divisible by 2, 13 as well as 1 and itself therefore 26 cannot be a prime number

27

Divisible by 2, 3 and 9 as well as 1 and itself, so 27 isn't a prime number either

28

Divisible by 2, 4, 7 and 14 as well as 1 and itself, therefore 28 is not a prime number

29

Divisible only by 1 and itself, therefore 29 is a prime number.


What you are looking for is factors of the number other than 1 and the number itself, if you cannot find any then you are dealing with a prime number.


See also "Prime Factors Tree" in the topic "Surds"


Questions that you are asked to may not always be as cold and clinical as "find the factors in this set of numbers" you may be asked to apply a little bit of existing knowledge to answer a question.


>> Questions <<