BREAKING: Target Test Prep releases Brand New 2026 On Demand GMAT prep course

Redeem

Is there a faster way to answer this GMAT question?

This topic has expert replies
Post new topic Post Reply
Previous Topic Next Topic
Newbie | Next Rank: 10 Posts
Posts: 2
Joined: Tue Jun 18, 2013 7:48 am
Followed by:1 members

Is there a faster way to answer this GMAT question?

by SBronson » Tue Jun 18, 2013 10:02 pm
There's gotta be a faster way to answer this question? Can anyone help?

117. If n is positive, which of the following is equal to:

1/(Rad (n+1)− Rad n)?
(A) 1
(B) Rad (2n+1)
(C) Rad (n+1)/Rad n
(D) Rad (n+1) − rad n
(E) Rad (n+1) + Rad n

The official guide explanation is about half a page long.

Thanks!

-Samantha
Top
Source: — Problem Solving |
User avatar
GMAT Instructor
Posts: 15539
Joined: Tue May 25, 2010 12:04 pm
Location: New York, NY
Thanked: 13060 times
Followed by:1906 members
GMAT Score:790

by GMATGuruNY » Tue Jun 18, 2013 11:54 pm
Image

Let n = 2.
Then:
1/(√n+1 - √n) = 1/(√3-√2) ≈ 1/(1.7 - 1.4) = 1/.3 = 10/3. This is our target.

Now plug n=2 into the answers to see which comes closest to our target of 10/3.

Only E works:
√(n+1) + √n = √3+√2 ≈ 1.7 + 1.4 = 3.1.

The correct answer is E.
Private tutor exclusively for the GMAT and GRE, with over 20 years of experience.
Followed here and elsewhere by over 1900 test-takers.
I have worked with students based in the US, Australia, Taiwan, China, Tajikistan, Kuwait, Saudi Arabia -- a long list of countries.
My students have been admitted to HBS, CBS, Tuck, Yale, Stern, Fuqua -- a long list of top programs.

As a tutor, I don't simply teach you how I would approach problems.
I unlock the best way for YOU to solve problems.

For more information, please email me (Mitch Hunt) at [email protected].
Student Review #1
Student Review #2
Student Review #3
Top
Newbie | Next Rank: 10 Posts
Posts: 4
Joined: Mon Jun 17, 2013 11:02 pm

by Raj700 » Wed Jun 19, 2013 5:38 am
Nice! Thank you Mitch. That's so much easier. It certainly helps though to have rad 2 and rad 3 memorized.

-Raj
Top
Newbie | Next Rank: 10 Posts
Posts: 1
Joined: Mon Feb 18, 2013 4:40 pm

by Ashke » Wed Jun 19, 2013 8:11 am
1/(Rad (n+1)- Rad n)

= (Rad (n+1)+ Rad n)/((Rad (n+1)- Rad n))*(Rad (n+1)+ Rad n))

= (Rad (n+1)+ Rad n)
Top
User avatar
Master | Next Rank: 500 Posts
Posts: 104
Joined: Fri Oct 07, 2011 10:23 pm
Thanked: 36 times
Followed by:4 members

by fcabanski » Wed Jun 19, 2013 10:41 pm
Here's an even faster answer:

"OnlyEworks"

Seriously, though, plugging in is often faster than working the algebra. But if you remember how to rationalize the denominator, as Ashke showed, the algebra on this one takes about 10 seconds.
Expert GMAT tutor.
[email protected]

If you find one of my answers helpful, please click thank.

Contact me to discuss online GMAT tutoring.
Top
Post new topic Post Reply
• Page 1 of 1