Find all School-related info fast with the new School-Specific MBA Forum

It is currently 01 Oct 2014, 00:23

Close

GMAT Club Daily Prep

Thank you for using the timer - this advanced tool can estimate your performance and suggest more practice questions. We have subscribed you to Daily Prep Questions via email.

Customized
for You

we will pick new questions that match your level based on your Timer History

Track
Your Progress

every week, we’ll send you an estimated GMAT score based on your performance

Practice
Pays

we will pick new questions that match your level based on your Timer History

Not interested in getting valuable practice questions and articles delivered to your email? No problem, unsubscribe here.

Events & Promotions

Events & Promotions in June
Open Detailed Calendar

Prime factor questions

  Question banks Downloads My Bookmarks Reviews Important topics  
Author Message
TAGS:
Intern
Intern
avatar
Joined: 27 Jul 2007
Posts: 3
Followers: 0

Kudos [?]: 0 [0], given: 0

Prime factor questions [#permalink] New post 27 Jul 2007, 12:09
Hey all, i am having difficulty getting a hang of the prime factor questions. would be very glad if someone could show me the thought process to solve the following question:

What is the greatest prime factor of 4^17 - 2^28 ?
- 2
- 3
- 5
- 7
- 11


Also, although not as important:
Why is, if x<0, sqrt(-x|x|) = -x?
Is the sqrt of -4 equal to -2?

Many thanks for your help, Max
Director
Director
User avatar
Joined: 26 Feb 2006
Posts: 908
Followers: 3

Kudos [?]: 39 [0], given: 0

Re: Prime factor questions [#permalink] New post 27 Jul 2007, 12:17
MaxCR1 wrote:
Hey all, i am having difficulty getting a hang of the prime factor questions. would be very glad if someone could show me the thought process to solve the following question:

What is the greatest prime factor of 4^17 - 2^28 ?
- 2
- 3
- 5
- 7
- 11



= 4^17 - 2^28
= 2^34 - 2^28
= 2^28 (2^6 -1)
= 2^28 (63)

now it should be 7.
Director
Director
User avatar
Joined: 26 Feb 2006
Posts: 908
Followers: 3

Kudos [?]: 39 [0], given: 0

Re: Prime factor questions [#permalink] New post 27 Jul 2007, 12:19
MaxCR1 wrote:
Why is, if x<0, sqrt(-x|x|) = -x?. Is the sqrt of -4 equal to -2?



sqrt of -4 is not equal to -2 but (-sqrt of 4) is equal to -2.
Intern
Intern
avatar
Joined: 27 Jul 2007
Posts: 3
Followers: 0

Kudos [?]: 0 [0], given: 0

 [#permalink] New post 27 Jul 2007, 12:49
many thanks for your promp help, i haven't done maths in quite a while..
i understand it to this part:
= 2^34 - 2^28

but i am unsure of the logic of substracting 1 from the 2^6:
= 2^28 (2^6 -1)
= 2^28 (63)

Is the formula:
a^b - a^c = a^c*[a^(a-c)-1]

What would it be for
a^b + a^c ?

Many thanks for your help
Director
Director
User avatar
Joined: 26 Feb 2006
Posts: 908
Followers: 3

Kudos [?]: 39 [0], given: 0

 [#permalink] New post 27 Jul 2007, 13:10
MaxCR1 wrote:
many thanks for your promp help, i haven't done maths in quite a while..
i understand it to this part:
= 2^34 - 2^28

but i am unsure of the logic of substracting 1 from the 2^6:
= 2^28 (2^6 -1)
= 2^28 (63)

Is the formula:
a^b - a^c = a^c*[a^(a-c)-1]

What would it be for
a^b + a^c ?

Many thanks for your help


a^b - a^c = a^c [a^(b-c) - 1] for b>c
a^b + a^c = a^c (a^(b-c) + 1) for b>c
  [#permalink] 27 Jul 2007, 13:10
    Similar topics Author Replies Last post
Similar
Topics:
1 Experts publish their posts in the topic Question Regarding Multiples, factors and primes! SonyGmat 6 09 Nov 2011, 15:11
1 From MitDavidDv: Prime Factor Question MitDavidDv 3 01 Jun 2011, 12:10
Number properties question - prime factors chloeholding 3 01 May 2011, 00:51
Exponents / prime factors question jthong 7 07 Oct 2007, 16:43
Prime factor sandalphon 10 20 Dec 2005, 07:59
Display posts from previous: Sort by

Prime factor questions

  Question banks Downloads My Bookmarks Reviews Important topics  


GMAT Club MBA Forum Home| About| Privacy Policy| Terms and Conditions| GMAT Club Rules| Contact| Sitemap

Powered by phpBB © phpBB Group and phpBB SEO

Kindly note that the GMAT® test is a registered trademark of the Graduate Management Admission Council®, and this site has neither been reviewed nor endorsed by GMAC®.