Greatest Prime factor

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by GMATGuruNY » Sun Jul 14, 2013 6:06 pm
Nirupam04 wrote:hi, Kindly help to solve this question.

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

(4 to the power 7)- (2 to the power 28)
A> 2
B> 3
C> 5
D> 7
E> 11
Rephrase the expression in terms of a COMMON BASE:
(2²)¹� - 2²�
2³� - 2²�.

Factor out the GREATEST COMMON FACTOR and SIMPLIFY:
2²�(2� - 1)
2²�(63).

PRIME-FACTORIZE:
2²�(63) = 2²� * 3² * 7.

The greatest prime factor is the value in red.

The correct answer is D.
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