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# What is the greatest prime factor of 4^17 - 2^28 ? 2 3 5 7

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Manager
Joined: 28 Aug 2008
Posts: 95
What is the greatest prime factor of 4^17 - 2^28 ? 2 3 5 7 [#permalink]

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14 Sep 2008, 19:45
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What is the greatest prime factor of 4^17 - 2^28 ?

2
3
5
7
11

--== Message from GMAT Club Team ==--

This is not a quality discussion. It has been retired.

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Director
Joined: 14 Aug 2007
Posts: 695
Re: PS - Number Props [#permalink]

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14 Sep 2008, 19:54
4^17 - 2^28
=2^34 - 2^28
= 2^28 * ( 2^6 - 1)
= 2^28 * (63)
= 2^28 * 7 * 3 * 3

7 is the greatest prime factor

D
Manager
Joined: 28 Aug 2008
Posts: 95
Re: PS - Number Props [#permalink]

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14 Sep 2008, 20:02
Correct

Can you please explain how to go from:

2^34 - 2^38 to 2^28 * (2^6 - 1 )?

thx
Director
Joined: 14 Aug 2007
Posts: 695
Re: PS - Number Props [#permalink]

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14 Sep 2008, 20:40
IgnitedMind wrote:
Correct

Can you please explain how to go from:

2^34 - 2^28 to 2^28 * (2^6 - 1 )?

thx

sure,

2^34 = 2^28 * 2^6
so,
2^34 - 2^28 = (2^28 * 2^6) - 2^28
take 2^28 common,
this will give u,
2^28 * (2^6 - 1 )

--== Message from GMAT Club Team ==--

This is not a quality discussion. It has been retired.

If you would like to discuss this question please re-post it in the respective forum. Thank you!

To review the GMAT Club's Forums Posting Guidelines, please follow these links: Quantitative | Verbal Please note - we may remove posts that do not follow our posting guidelines. Thank you.
Re: PS - Number Props   [#permalink] 14 Sep 2008, 20:40
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# What is the greatest prime factor of 4^17 - 2^28 ? 2 3 5 7

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