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What is the greatest common factor of 4^17 - 2^28

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by truplayer256 » Wed Jul 29, 2009 3:57 pm
You're on the right track.

2^34-2^28

2^28(2^6-1)

2^28(63)

63 is divisible by the prime factor 7. Therefore, 7 is our answer.
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by prindaroy » Wed Jul 29, 2009 4:00 pm
= 2^34 - 2^28

= 2^28(2^6 - 1)

= 2^28(64-1)

= 2^28(63)

7|63, so 7 is the answer
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by doclkk » Wed Jul 29, 2009 8:39 pm
truplayer256 wrote:You're on the right track.

2^34-2^28

2^28(2^6-1)

2^28(63)

63 is divisible by the prime factor 7. Therefore, 7 is our answer.
I knew it.

darn.

I know how to do that - i've done that a million times. darn darn it.
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Re: What is the greatest common factor of 4^17 - 2^28

by real2008 » Wed Jul 29, 2009 11:41 pm
doclkk wrote:What is the greatest common factor of

4^17 - 2^28

2
3
5
7
11

OA: D

I broke this down into 2^34 - 2^28

What's next step?
R U sure is this the question?
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by Ian Stewart » Thu Jul 30, 2009 4:57 am
The question should read "what is the greatest prime factor of 4^17 - 2^28". We only ever talk about 'common factors' when we are talking about two or more numbers. This question is about a single number.
For online GMAT math tutoring, or to buy my higher-level Quant books and problem sets, contact me at ianstewartgmat at gmail.com

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