2 gmat prep 2 questions

This topic has expert replies
Junior | Next Rank: 30 Posts
Posts: 22
Joined: Fri Jul 04, 2008 11:13 am
Followed by:1 members

2 gmat prep 2 questions

by JDesai01 » Sat Jul 26, 2008 6:16 pm
1. (sqroot(9 + sqroot(80)) + sqroot(9 - sqroot(80)))^2 = ?

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

Thank you

Junior | Next Rank: 30 Posts
Posts: 22
Joined: Fri Jul 04, 2008 11:13 am
Followed by:1 members

by JDesai01 » Sun Jul 27, 2008 7:18 am
Figured out the first one..

Junior | Next Rank: 30 Posts
Posts: 22
Joined: Fri Jul 04, 2008 11:13 am
Followed by:1 members

by JDesai01 » Sun Jul 27, 2008 7:57 am
And now got the 2nd one.. sorry for the bother. Thanks

Newbie | Next Rank: 10 Posts
Posts: 2
Joined: Wed Feb 27, 2008 4:13 am

by mkingsly » Tue Jul 29, 2008 11:30 am
Hi,

Can you post the answer pls?
I am getting 20 for the question1 and (2^28).(3^2).7 for question2.

Thanks,
Kingsly.

Junior | Next Rank: 30 Posts
Posts: 22
Joined: Fri Jul 04, 2008 11:13 am
Followed by:1 members

by JDesai01 » Tue Jul 29, 2008 11:39 am
1. 20
2. 7

Newbie | Next Rank: 10 Posts
Posts: 3
Joined: Sun Jul 20, 2008 6:19 pm

pls post the soln

by karthikeyan.sivanantham » Wed Jul 30, 2008 4:52 pm
guys,
Can you please post the solution, will be much appreciated.

Thanks,
karthik

User avatar
GMAT Instructor
Posts: 85
Joined: Thu May 01, 2008 12:56 pm
Location: Salt Lake City, UT
Thanked: 24 times
GMAT Score:750+

by VP_RedSoxFan » Thu Jul 31, 2008 8:45 am
Here's my unsolicited help on number 2 since I think it is such a classic GMAT problem. Since no one asked for help, feel free to tell me to shut up :).

4^17 - 2^28 = (2^2)^17 - 2^28 = 2^34 - 2^28 from here, factor out a 2^28 leaving you with:

2^28 * (2^6 - 1)

Since we know the 2^28 portion just really represents 28 factors of 2, we know that those are not the highest prime factors of this product. If we solve for 2^6 - 1, we get 63 whose prime factors are 3, 3, 7.

7 is therefore the greatest prime factor of this number (the other prime factors are 3, 3, and 28 2's).
Ryan S.
| GMAT Instructor |
Elite GMAT Preparation and Admissions Consulting
www.VeritasPrep.com

Learn more about me

Newbie | Next Rank: 10 Posts
Posts: 9
Joined: Fri Feb 08, 2008 1:26 pm
Location: Dallas

by LSUKnowIt » Sat Aug 02, 2008 10:18 am
2^28 * (2^6 - 1)


I understand where the 2^6 comes from but can someone please explain to me where the -1 comes from????

Thanks

Senior | Next Rank: 100 Posts
Posts: 69
Joined: Fri Jul 11, 2008 8:49 am
Thanked: 1 times

by uncbeers » Sat Aug 02, 2008 10:31 am
If you search there are really good explanations of this question but I will try to help you.

4^17 = (2^2)^17 = 2^34

2^34 - 2^28 = Factor from here!

2^28 * (2^6 - 1) (If you were to multiply this out it goes right back to the form above)

Does this help?

Newbie | Next Rank: 10 Posts
Posts: 9
Joined: Fri Feb 08, 2008 1:26 pm
Location: Dallas

by LSUKnowIt » Sat Aug 02, 2008 11:12 am
I appriciate the help but i still dont get it.

34-28 is 6 so I think that's where the 6 comes from but that -1 is throwing me off still. It's just not clicking for me for some reason

Newbie | Next Rank: 10 Posts
Posts: 9
Joined: Fri Feb 08, 2008 1:26 pm
Location: Dallas

by LSUKnowIt » Sat Aug 02, 2008 11:14 am
I appriciate the help but i still dont get it.

34-28 is 6 so I think that's where the 6 comes from but that -1 is throwing me off still. It's just not clicking for me for some reason

Newbie | Next Rank: 10 Posts
Posts: 1
Joined: Sat Aug 02, 2008 11:31 am

by nyparis1 » Sat Aug 02, 2008 11:34 am
Could someone pls advise on the first Q?
1. (sqroot(9 + sqroot(80)) + sqroot(9 - sqroot(80)))^2 = ?

Thanks

Senior | Next Rank: 100 Posts
Posts: 69
Joined: Fri Jul 11, 2008 8:49 am
Thanked: 1 times

by uncbeers » Sat Aug 02, 2008 12:43 pm
LSU I sent you a PM with a few more examples and steps in it.

nyparis- there are some really good examples of that problem being solved in other threads. The answer can get quite long if all steps are written out so my advce is to do a search for '(sqroot(9 + sqroot(80))' or something similar.

EDIT: Here is one of those threads: https://www.beatthegmat.com/sq-roots-t12808.html

Senior | Next Rank: 100 Posts
Posts: 79
Joined: Sat May 24, 2008 8:57 pm
Location: O-Town FL
Thanked: 1 times

-1?

by LifetimesofSC » Sun Aug 10, 2008 11:55 am
I too am lost at the -1.

I would like some examples regarding that step.

Thanks