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# m06 #13

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Director
Joined: 25 Oct 2008
Posts: 608
Location: Kolkata,India
Followers: 13

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17 Sep 2009, 03:02
Is P-1 always even?

P is prime.
P=2k+1, where K is any positive integer.

Guys,this is a yes/no question and I got a bit confused by the word "always" in the question.Please correct me:

Stmt 1: If P=2,P-1 =odd
If P is any other prime P-1 will be odd.
Hence, NO P-1 IS NOT ALWAYS EVEN

Checking for A/D

Stmt 2:Similar process.NO P-1 IS NOT ALWAYS EVEN
ANS:D

But if the word "always" was'nt there we can safely say that the OA is correct..
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Joined: 29 Aug 2009
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17 Sep 2009, 14:39
1)
P is prime
2-1= 1 odd
3-1=2 even
5-1=4 even

even / odd cant say for sure. So check for BCE

2) P=2k+1 where k is positive integer
so P-1=2K
2 multiplied by any number odd/even is odd
hence P-1 is always going to be even

So Ans is B
Re: m06 #13   [#permalink] 17 Sep 2009, 14:39
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# m06 #13

Moderator: Bunuel

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