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# What is the prine factor of 4^17 - 2^28 ? 2 3 5 7 11

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What is the prine factor of 4^17 - 2^28 ? 2 3 5 7 11  [#permalink]  23 Jun 2007, 14:39
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What is the prine factor of 4^17 - 2^28 ?

2
3
5
7
11

i'm stumped ...
Director
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Re: GmatPrep prime factor PS [#permalink]  23 Jun 2007, 15:22
overtherainbow wrote:
What is the prine factor of 4^17 - 2^28 ?

2
3
5
7
11

i'm stumped ...

4^17=(2^2)17=2^34

The question now becomes what is the (greatest) prime factor of:
2^34-2^28 = 2^28(2^6-1) = 2^28*63

prime factor of 2^28=2
prime factors of 63= 3 & 7

Therefore, the greatest prime factor of 2^28*63 = 7

Re: GmatPrep prime factor PS   [#permalink] 23 Jun 2007, 15:22
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