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GMAT in 3 days! Experts please help explain answer

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by sharpdresser » Sat Jan 05, 2013 12:02 am
First let me say thank you all for helping me in the past 3 weeks! I spent each day on these forums reading every post and comment that was available, increasing my practice score from 660 to 740. I decided to make a username for these last few days to seek help on a few questions. I will greatly appreciate any advice on my posts, and I promise to always use spoilers!


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by sharpdresser » Sat Jan 05, 2013 12:18 am
OA D-7
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by sharpdresser » Sat Jan 05, 2013 1:48 am
OA D-7
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by Param800 » Sat Jan 05, 2013 3:46 am
Hello There,

I did this question this way--

4^17-2^28 = 2^34-2^28

Now factor out 2^28 from the above expression...

2^28 ( 2^6 -1 ) = 2^28 * 63

63 can be written as 7*3*3

...So from above we know that the greatest prime factor is 7.

Hope this helps. And BEST OF LUCK for your Exam. :)
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by ceilidh.erickson » Sat Jan 05, 2013 7:47 am
Param800 wrote:Hello There,

I did this question this way--

4^17-2^28 = 2^34-2^28

Now factor out 2^28 from the above expression...

2^28 ( 2^6 -1 ) = 2^28 * 63

63 can be written as 7*3*3

...So from above we know that the greatest prime factor is 7.

Hope this helps. And BEST OF LUCK for your Exam. :)
This is absolutely correct. Just a general piece of advice with any exponent problems like this (particularly ones that mention "factors"): always start by getting down to prime factorization. Convert 4 to 2^2, etc. This will allow you to get equal bases, and then to simplify.

Good luck!
Ceilidh Erickson
EdM in Mind, Brain, and Education
Harvard Graduate School of Education
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