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**mandy jane****Member**- Registered: 2010-09-23
- Posts: 1,003

Hi stefy mj here question 1 (d) write down an explanation as to how you found these numbers?

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**bob bundy****Moderator**- Registered: 2010-06-20
- Posts: 6,532

hi Mandy

prime numbers

You have to know your times tables a bit.

These are **not **prime numbers

6 = 2 x 3

8 = 2 x 4

15 = 3 x 5

28 = 4 x 7

88 = 8 x 11

But if you cannot make a multiplication sum out of a number then it is prime.

**Here are some prime numbers:**

**2 3 5 7 **

What do you think is the next prime number?

Bob

You cannot teach a man anything; you can only help him find it within himself..........Galileo Galilei

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**mandy jane****Member**- Registered: 2010-09-23
- Posts: 1,003

is the next prime number 9 then? send asnwer back now?

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**bob bundy****Moderator**- Registered: 2010-06-20
- Posts: 6,532

Well I see why you said this but hang on

9 = 3 x 3 so 9 is **not prime.**

Please will you try again?

Bob

You cannot teach a man anything; you can only help him find it within himself..........Galileo Galilei

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**mandy jane****Member**- Registered: 2010-09-23
- Posts: 1,003

is it 8 then? let me know?

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**bob bundy****Moderator**- Registered: 2010-06-20
- Posts: 6,532

These are not prime numbers

6 = 2 x 3

8 = 2 x 4

15 = 3 x 5

28 = 4 x 7

88 = 8 x 11

8 is in the list of examples that are not prime.

So not 8, not 9.

What do you think is the next prime number?

Bob

You cannot teach a man anything; you can only help him find it within himself..........Galileo Galilei

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**mandy jane****Member**- Registered: 2010-09-23
- Posts: 1,003

is it 11 then? let me know please?

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**anonimnystefy****Real Member**- From: The Foundation
- Registered: 2011-05-23
- Posts: 15,658

That is correct. Do you know how you could check if it is a prime?

Here lies the reader who will never open this book. He is forever dead.

Taking a new step, uttering a new word, is what people fear most. ― Fyodor Dostoyevsky, Crime and Punishment

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**mandy jane****Member**- Registered: 2010-09-23
- Posts: 1,003

hi stefy how do you check if it is a prime number then? send me asnswer now please?

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**bob bundy****Moderator**- Registered: 2010-06-20
- Posts: 6,532

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**anonimnystefy****Real Member**- From: The Foundation
- Registered: 2011-05-23
- Posts: 15,658

You could check if it was divisible by any number between 1 and 11 not including them,but it is easier to check divisibility by every number to the approximate square root of the number. E.g.:

sqrt(11)=3.something,where something represents the decimals. This means we check for all numbers from 2 to 3,that is,you check divisibility by 2 and 3 only.

Can you work out if 53 is a prime?

Here lies the reader who will never open this book. He is forever dead.

Taking a new step, uttering a new word, is what people fear most. ― Fyodor Dostoyevsky, Crime and Punishment

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**mandy jane****Member**- Registered: 2010-09-23
- Posts: 1,003

hi bob bundy yes i found this page ok? what should i do next then? send message now please?

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**bob bundy****Moderator**- Registered: 2010-06-20
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look at my picture

12 has factors.

Will you try 13?

Bob

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**mandy jane****Member**- Registered: 2010-09-23
- Posts: 1,003

hi bob bundy 13 has no factors am i right or not?let me know now?

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**bob bundy****Moderator**- Registered: 2010-06-20
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You should have found that 13 has no factors.

So it is **prime.**

What is the next prime after 13?

Bob

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**mandy jane****Member**- Registered: 2010-09-23
- Posts: 1,003

hi stefy 53 is not a prime number am i right or not?let me know?

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**bob bundy****Moderator**- Registered: 2010-06-20
- Posts: 6,532

What factors did you find for 53?

Bob

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**bob bundy****Moderator**- Registered: 2010-06-20
- Posts: 6,532

look at the picture.

Bob

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**mandy jane****Member**- Registered: 2010-09-23
- Posts: 1,003

Hi bob bundy the next prime number after 13 is 17 am i right? and the factor for 53 is there is not one am i right?

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**bob bundy****Moderator**- Registered: 2010-06-20
- Posts: 6,532

Well done. !7 is correct.

but look at my picture for 53.

Do you still think it is not one?

Bob

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**mandy jane****Member**- Registered: 2010-09-23
- Posts: 1,003

hi bob bundy i am not sure? let me know? what should i do now?

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**bob bundy****Moderator**- Registered: 2010-06-20
- Posts: 6,532

Now you know how to find out if a number is prime here is some homework:

The list of primes starts

2, 3, 5, 7, 11, 13, 17, ........., 53, .........97, ..........

Use the prime factorization tool to find **all the primes from 17 up to 97.**

You don't need to try every number since it should be obvious some numbers have factors

eg. 70 ends in a zero so it is 10 x 7

Do you think you can do this homework for me by tomorrow morning?

Bob

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**mandy jane****Member**- Registered: 2010-09-23
- Posts: 1,003

Hi bob bundy here is my home work for you prime number are 19 , 59, 101 then? and the prime numbers beween

17 - 97 are 19, 23, 29, 31, 37, 31, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89 let me know if i am right or not please? Then let me know what to do next? I will look tomorrow ok?

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**anonimnystefy****Real Member**- From: The Foundation
- Registered: 2011-05-23
- Posts: 15,658

Hi MJ

Yes,thos are the primes between 17 and 90,but you didn't check the numbers between 90 and 97. Can you do that,just to make sure you get it?

Here lies the reader who will never open this book. He is forever dead.

Taking a new step, uttering a new word, is what people fear most. ― Fyodor Dostoyevsky, Crime and Punishment

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**bob bundy****Moderator**- Registered: 2010-06-20
- Posts: 6,532

Hi Mandy,

That's really great. Well done.

Let's do one more thing with prime numbers.

It will help to make the idea really clear for you.

Below I've started a picture which will show every prime below 100.

Now you already know these so it will be a good way to practise the ideas.

What you do is** shade all the squares that are not primes.**

What is left un-shaded should be just the list of primes.

Step one. Print off the picture or make a copy using square paper.

Step two.

Shading everything in the 2x table. Now we know that **2 is a prime** so don't make a mistake and shade 2 itself.

But everything else in the 2x table will have 2 as a factor.

eg. 4 = 2x2

6 = 2x3

8 = 2x4 and so on.

So I've shaded some of the 2x table in yellow. Your job is to finish off all the 2s in yellow.

Step three.

Shading everything in the 3x table. Now we know that **3 is a prime** so don't make a mistake and shade 3 itself.

Also, some of the 3x table has already been shaded yellow.

eg 6 = 2x3 is in both 2 and 3 x tables.

So you only need to shade the ones like

9 = 3x3 and 15 = 3x5 and so on. I've used orange for these. Your job is to shade every 3rd number to finish off all the 3s in orange.

Step four.

The 5x table. (Can you see why I've skipped the 4x table?)

As usual don't shade 5 itself, but very 5th number after that, if it isn't already shaded. I've suggested blue for these.

Step five.

The 7x table. (Can you see why I've skipped the 6x table?)

As usual don't shade 7 itself, but very 7th number after that, if it isn't already shaded. I've suggested green for these.

By now you should have only the primes left unshaded. Take care because it is hard to unshade if you make a mistake.

The satisfying thing will be doing this yourself and ending up with the same list you have already.

Good luck with it,

Bob

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