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

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Intern
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What is the greatest prime factor of 4^17 - 2^28 [#permalink]

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New post 14 Dec 2008, 12:09
This topic is locked. If you want to discuss this question please re-post it in the respective forum.

Can someone provide the answer (and explaination) please.

Thanks in advance...

What is the greatest prime factor of 4^17 - 2^28?
a) 2
b) 3
c) 5
d) 7
e) 11

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Re: GMAC question [#permalink]

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New post 14 Dec 2008, 12:13
Never mind - I got it. factor out the 2^34.

Thanks anyway.

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Re: GMAC question [#permalink]

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New post 14 Dec 2008, 19:38
d)

2^28(2^6 - 1)
2^28 * 63

prime factors 2,3,7

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Re: GMAC question [#permalink]

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New post 15 Dec 2008, 06:27
gmatatouille wrote:
d)

2^28(2^6 - 1)
2^28 * 63

prime factors 2,3,7


gmatatouille - thanks for the bolded formula! Now I know what's the quickest way to calculate it ;)

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Re: GMAC question [#permalink]

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New post 15 Dec 2008, 09:46
What is the greatest prime factor of 4^17 - 2^28?
a) 2
b) 3
c) 5
d) 7
e) 11

2^34 - 2^28 = 2^28(2^6 -1) = 64-1 = 63 = 3^2*7..........answer is 7

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Re: GMAC question   [#permalink] 15 Dec 2008, 09:46
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What is the greatest prime factor of 4^17 - 2^28

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