Greatest prime factor

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fltingley: Senior | Next Rank: 100 Posts; Posts: 47; Joined: Mon Oct 02, 2006 4:19 am; Thanked: 3 times

by fltingley » Wed May 27, 2009 7:11 pm

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

2
3
4
7
11

MGCAT answer: D

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Source: Beat The GMAT — Problem Solving | Wed May 27, 2009 7:11 pm

dumb.doofus: Master | Next Rank: 500 Posts; Posts: 435; Joined: Sat Sep 27, 2008 2:02 pmFollowed by: 1 member; Location: San Jose, CA; Thanked: 43 times; GMAT Score:720

by dumb.doofus » Wed May 27, 2009 7:19 pm

2^34 - 2^28 = 2^28(2^6 - 1) = 2^28(63) = 2^28(9*7)

therefore 7

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