greatest prime factor

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dmcreavy: Newbie | Next Rank: 10 Posts; Posts: 5; Joined: Wed Feb 11, 2009 7:57 am

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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Source: Beat The GMAT — Problem Solving | Mon Mar 02, 2009 2:29 pm

praxis: Junior | Next Rank: 30 Posts; Posts: 26; Joined: Thu Apr 03, 2008 1:20 pm; Thanked: 6 times

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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dmcreavy: Newbie | Next Rank: 10 Posts; Posts: 5; Joined: Wed Feb 11, 2009 7:57 am

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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praxis: Junior | Next Rank: 30 Posts; Posts: 26; Joined: Thu Apr 03, 2008 1:20 pm; Thanked: 6 times

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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Baldini: Senior | Next Rank: 100 Posts; Posts: 90; Joined: Mon Mar 02, 2009 6:06 am; Thanked: 3 times

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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praxis: Junior | Next Rank: 30 Posts; Posts: 26; Joined: Thu Apr 03, 2008 1:20 pm; Thanked: 6 times

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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praxis: Junior | Next Rank: 30 Posts; Posts: 26; Joined: Thu Apr 03, 2008 1:20 pm; Thanked: 6 times

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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caludios: Newbie | Next Rank: 10 Posts; Posts: 3; Joined: Wed Oct 22, 2008 7:24 am

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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