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greatest prime factor

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Newbie | Next Rank: 10 Posts
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by dmcreavy » Mon Mar 02, 2009 2:29 pm
What is the greatest prime factor of 4^17 - 2^28?
A. 2
B. 3
C. 5
D. 7
E. 11

Help!!! I get to 2^34 - 2^28 and don't know where to go from here...

Thanks! David
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Junior | Next Rank: 30 Posts
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The trick is to factor out the subtraction...

by praxis » Mon Mar 02, 2009 2:48 pm
2^34 - 2^28 = 2^6 * 2^28 - 2*28 = (2^6 - 1)*2^28

Now,

2^6 - 1 = 64 - 1 = 63 = 7*3*3

and the greatest prime factor is 7.

BTW, for some practice questions like this you can visit
https://www.testsandtutors.com/course/qu ... -and-Roots
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by dmcreavy » Mon Mar 02, 2009 3:03 pm
Perfect thank you for the additional link...I knew I was missing something easy...
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by praxis » Tue Mar 03, 2009 10:39 am
You're welcome - the large exponents can be intimidating, but remember without a calculator, they're not going to make you really calculate very large numbers. There's usually a trick!
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by Baldini » Thu Mar 05, 2009 11:28 am
excuse me praxis, just wondering if you could help.
I went on to your link, signed up etc, started doing the section on exponentials and after answering the first question right, can not go any further (in other words, the site will not let me see the other questions). Do you have any idea why?

thx
GMAX
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by praxis » Mon Mar 09, 2009 9:52 am
That's very odd; the site shows at least 8 questions in that category. I'll see how long it takes to get fixed!
Want more practice GMAT questions? check out
https://www.testsandtutors.com/course/view.php/GMAT
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by praxis » Mon Mar 09, 2009 11:01 am
A bunch of the categories were not displaying all the available questions, this is fixed!
Want more practice GMAT questions? check out
https://www.testsandtutors.com/course/view.php/GMAT
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HELP FOR EXPONENTS PROBLEMS!!!

by caludios » Mon Mar 30, 2009 2:53 am
Hello,

even if i read many PS problems anout Exponents, I still cannot understand how to solve them. Specially with high root and way to sumplify an equation.
I do need a guide, and not exercises. Who can help me?
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