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What is the greatest prime factor ?

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by kmittal82 » Mon Sep 12, 2011 6:16 am
4^17 = 2^34

The question stem then can be simplified:

2^34 - 2^28
= 2^28(2^6 - 1)
= 2^28 x 63

7 is the greatest prime factor for 63 (7x9 = 63), hence 7 is the answer
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by navami » Mon Sep 12, 2011 9:59 am
yes 7 in the ans
This time no looking back!!!
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by Abhishek009 » Mon Sep 12, 2011 10:11 am
Ashetty wrote:What is the greatest prime factor of 4^17 - 2^28?
A.2
B.3
C.5
D.7
E.11

Ans:7

Thanks
Just the other way :

4^17 - 2^28

=> 4^17 - 4^14

=> 4^14 ( 4^3 - 1 )

=> 4^14 ( 64 - 1 )

=> 4^14 ( 63 )

=> 2^28 * ( 7 ) * ( 3 )^2

=> Hence we can certainly say that the greatest prime factor among the three number 2 , 3 and 7 is 7....


So , Answer will be 7...
Abhishek
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