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by **guest** Thu Aug 02, 2007 7:17 pm

Can anyone help answer the following question: What is the greatest prime factor of 4^17-2^28?

The answer is 7 and I don't know why...

by **dbernst** Fri Aug 03, 2007 1:27 am

4^17-2^28
= (2^2)^17 - 2^28
= 2^34 - 2^28
Now, factor out 2^28
= 2^28(2^6 - 1)
Since 2^6 - 1 = 63 = 3*3*7, the greatest prime factor = 7

-dan

Can anyone help answer the following question: What is the greatest prime factor of 4^17-2^28?

The answer is 7 and I don't know why...