Check GMAT Club Decision Tracker for the Latest School Decision Releases https://gmatclub.com/AppTrack

 It is currently 24 May 2017, 18:56

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

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

Events & Promotions

Events & Promotions in June
Open Detailed Calendar

If a and b are positive, is (a^-1+b^-1)^-1 less than (a^-1

Author Message
Senior Manager
Joined: 29 Aug 2005
Posts: 276
Followers: 2

Kudos [?]: 61 [0], given: 0

If a and b are positive, is (a^-1+b^-1)^-1 less than (a^-1 [#permalink]

Show Tags

12 Jun 2008, 00:05
00:00

Difficulty:

(N/A)

Question Stats:

0% (00:00) correct 0% (00:00) wrong based on 0 sessions

HideShow timer Statistics

This topic is locked. If you want to discuss this question please re-post it in the respective forum.

If a and b are positive, is (a^-1+b^-1)^-1 less than (a^-1 *b^-1)^-1?
(1) a = 2b
(2) a + b > 1
_________________

The world is continuous, but the mind is discrete

Director
Joined: 14 Oct 2007
Posts: 754
Location: Oxford
Schools: Oxford'10
Followers: 16

Kudos [?]: 218 [0], given: 8

Show Tags

12 Jun 2008, 02:14
vdhawan1 wrote:
If a and b are positive, is (a^-1+b^-1)^-1 less than (a^-1 *b^-1)^-1?
(1) a = 2b
(2) a + b > 1

is $$\frac{1}{\frac{1}{a}+\frac{1}{b}} < ab$$

which can be simplified to is b+a > 1

as seen 1) is insufficient and 2) is sufficient
CEO
Joined: 17 May 2007
Posts: 2959
Followers: 61

Kudos [?]: 601 [0], given: 210

Show Tags

12 Jun 2008, 02:28
Hi
$$\frac{1}{\frac{1}{a}+\frac{1}{b}} < ab$$

simplifies to
$$\frac{ab}{a+b} < ab$$
since $$a$$ and $$b$$ are positive we can safely multiply both sides by $$ab$$ and $$a+b$$

Hence it can be simplified to

$$a+b>1$$

B is sufficient.
Manager
Joined: 14 Jun 2007
Posts: 169
Location: Vienna, Austria
Followers: 1

Kudos [?]: 8 [0], given: 0

Show Tags

12 Jun 2008, 02:36
question to our experts: can anyone tell me why A is not sufficient?

cheers
Director
Joined: 14 Oct 2007
Posts: 754
Location: Oxford
Schools: Oxford'10
Followers: 16

Kudos [?]: 218 [0], given: 8

Show Tags

12 Jun 2008, 02:42
domleon wrote:
question to our experts: can anyone tell me why A is not sufficient?

cheers

after you simplify the question, it simplifies to is a+b > 1 ?

the stem says a and b are both positive, but it doesnt say that they are integers

so lets say if a=0.4 and b = 0.2 a+b < 1
but if a=6 and b = 3 a+b > 1

so 1) isn't sufficient
SVP
Joined: 28 Dec 2005
Posts: 1561
Followers: 3

Kudos [?]: 158 [0], given: 2

Show Tags

12 Jun 2008, 04:52
i said E because although stem says positive numbers, it doesnt mention that they have to be positive INTEGERS ...
Re: DS question   [#permalink] 12 Jun 2008, 04:52
Display posts from previous: Sort by