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 Post subject: prime and factorsPosted: Sun May 13, 2007 7:40 pm
hi,
My son is preparing for the 11+ exam in Essex.
I'd like to know if anyone know a easy method to learn prime numbers and factors
for agiven numbers

thnx

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 Post subject: Posted: Mon May 14, 2007 7:27 am

Joined: Thu Jan 12, 2006 3:29 pm
Posts: 625
Hi P&A

Assuming you are looking to write a number as a product of its primes.

A prime number has two distinct factors, itself and 1.

2 is the only even prime number, becasue it only has two factors, 2 and 1.

Any number can be written as a product of its primes.

First identify the prime numbers using eratosthenes sieve.

http://www.teachingideas.co.uk/maths/prime.htm

2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71
are the first twenty prime numbers.

Now choose at random any number, say 48

Start by dividing by 2

48 divided by 2 equal 24
24 divided by 2 equals 12
12 divided by 2 equals 6
6 divided by 2 equals 3

3 is a prime number so you cannot continue

48 can be written as 2 x 2 x 2 x 2 x 3, the product of its primes.

Try again with 63

63 is an odd number so cannot be divided by 2, so try 3

63 divided by 3 is 21
21 divided by 3 is 7
7 is a prime number so you cannot continue

63 can be written as 3 x 3 x 7, the product of its primes

Try again with 35

Can´t divide by 2, Can´t divide by 3, try 5

35 divided by 5 equals 7

7 is a prime number so you cannot continue

35 can be written as 5 x 7

Try 71

Can´t divide by any prime number except itself.

So 71, must be a prime number.

Keep practicing.

The product of primes is used to identify highest common factors and lowest common multiples of different numbers.

Hope this helps

Regards

Mike

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 Post subject: oops Mike.Posted: Mon May 14, 2007 5:38 pm
Hi Mike,

I was just looking at your demonstration of prime numbers and I noticed a small error.

63/3 does not equal 31. This is 21 which can then be divided again by three and this gives you 7 which is the the prime number.

Thanks

Jams Year5

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 Post subject: Posted: Tue May 15, 2007 10:44 am

Joined: Thu Jan 12, 2006 3:29 pm
Posts: 625
Hi Jammy

Thanks for picking that up. I have edited the text of the original posting so that it is now correct.

Regards

Mike

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