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# Factorials

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Intern
Joined: 23 May 2008
Posts: 47

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08 Oct 2008, 11:42
On pg 338 of the OG, question 153, explanation B it tries to explain the following:

"...2 is a factor of 13! + 2, since it is a factor of both 13! and 2..."

How do we know that 2 is a factor of 13!?

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Manager
Joined: 27 Sep 2008
Posts: 76

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08 Oct 2008, 12:21
easy !!

13!/2 = (1*2*3*4*5*6*7... 13)/2

so 13!/2 = integer

Intern
Joined: 23 May 2008
Posts: 47

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08 Oct 2008, 12:35
Sorry, still not getting it. I understand how to get to the factorial of 13! (1*2*3*4*5*6*7... 13). How is it common knowledge though that 2 is a factor of 13! without doing all the math?
Manager
Joined: 27 Sep 2008
Posts: 76

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08 Oct 2008, 12:39
snowbirdskier wrote:
Sorry, still not getting it. I understand how to get to the factorial of 13! (1*2*3*4*5*6*7... 13). How is it common knowledge though that 2 is a factor of 13! without doing all the math?

You will agree that 2&3 are factors of 6 since 6 can be written as 2*3 = 6 , same way 13!

Intern
Joined: 23 May 2008
Posts: 47

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08 Oct 2008, 12:44
Okay, you're right. That is easy. Thanks.

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Re: Factorials &nbs [#permalink] 08 Oct 2008, 12:44
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# Factorials

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