GMAT Question of the Day - Daily to your Mailbox; hard ones only

 It is currently 21 Jun 2018, 03:35

### 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
Manager
Joined: 28 Aug 2008
Posts: 95
If a and b are positive, is (a^-1 + b^-1)^-1 less than (a^-1 [#permalink]

### Show Tags

11 Sep 2008, 09:09
00:00

Difficulty:

(N/A)

Question Stats:

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

### HideShow timer Statistics

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

A
B
C
D
E

--== Message from GMAT Club Team ==--

This is not a quality discussion. It has been retired.

If you would like to discuss this question please re-post it in the respective forum. Thank you!

To review the GMAT Club's Forums Posting Guidelines, please follow these links: Quantitative | Verbal Please note - we may remove posts that do not follow our posting guidelines. Thank you.
Current Student
Joined: 28 Dec 2004
Posts: 3292
Location: New York City
Schools: Wharton'11 HBS'12

### Show Tags

11 Sep 2008, 09:28
so this simplifies to ab/(b+a) < ab?

since b and a are positive we can say

ab<ab(b+a)? which basically means its asking if a and b are fraction...and less than 1

1)

a=2b lets just pick number suppose b=3/7 then a=6/7

so 18/49 < (18/49)(9/7)?

yes..

suppose b=1 a=2 2<2(3) Yes

but suppose b=1/4 then a=1/2

1/8<1/8(3/4) NO...insuff

2)
a+b >1

Sufficient

IgnitedMind 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

VP
Joined: 17 Jun 2008
Posts: 1479

### Show Tags

11 Sep 2008, 10:20
Expression in the main question can be simplified to

is ab/(a+b) < ab......and since both a and b are positive, this further simplifies to
is (a+b) > 1........statement 2 clearly answers that.

Hence, B.
VP
Joined: 17 Jun 2008
Posts: 1289

### Show Tags

11 Sep 2008, 10:37
IgnitedMind 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

A
B
C
D
E

scthakur wrote:
ab/(a+b) < ab

question is :
ab/(a+b) < ab or ab(1/(a+b) -1 )<0 ????

(1) says a=2b , a=2b does not guarantee a+b>1 hence above expr can be +vbe or -ve INSUFFI

(2)a+b>1 does not say a and b have same signs or opposite ,in both cases different values for the expr.

(1) and (2) => SUFFI both are of same signs and again a+b>1 proves the relation given above

IMO C
_________________

cheers
Its Now Or Never

VP
Joined: 30 Jun 2008
Posts: 1008

### Show Tags

Updated on: 11 Sep 2008, 21:17
The best way to solve would be, simplify the question as much as possible

from the question we get to know that a and b are positive

so ab/(a+b) < ab can be written as ab(a+b) > ab and since ab is positive, we can divide both sides by ab ...

the simplified question is (a+b) > 1 ?

statement 1 says a = 2b ... this does not help me in telling whether a+b>1

incidentally statement 2 gives us the info directly that (a+b) > 1

so 2 is sufficient

_________________

"You have to find it. No one else can find it for you." - Bjorn Borg

Originally posted by amitdgr on 11 Sep 2008, 11:02.
Last edited by amitdgr on 11 Sep 2008, 21:17, edited 1 time in total.
Senior Manager
Joined: 31 Jul 2008
Posts: 270

### Show Tags

11 Sep 2008, 12:30
i think its B !

because we need either ab<0 (impossible as a N b both are +ve) OR a+b>1

first stamnt doesnt help 2b+b may or may not be >1

2 stmnt proves a+b>1

hence B
SVP
Joined: 07 Nov 2007
Posts: 1738
Location: New York

### Show Tags

11 Sep 2008, 12:49
IgnitedMind 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

A
B
C
D
E

ab/(a+b) < ab
Question is ab(a+b)>ab?

1) a=2b.. a+b can be <1 or a+b>1
not sufficient
2) a+b>1
also ab is positive.
ab(a+b)>ab is always true.
sufficient.

B
_________________

Smiling wins more friends than frowning

Manager
Joined: 09 Jul 2007
Posts: 236

### Show Tags

11 Sep 2008, 17:58
(a^-1 + b^-1)^-1 less than (a^-1 * b^-1)^-1 ?

simplifyling the expression ab/(a+b) < ab ? or is 1 < ( a+ b) , which is stated in option 2 directly

so IMO B
VP
Joined: 17 Jun 2008
Posts: 1289

### Show Tags

11 Sep 2008, 18:34
oops i didnt read the questio properly again!!!

--== Message from GMAT Club Team ==--

This is not a quality discussion. It has been retired.

If you would like to discuss this question please re-post it in the respective forum. Thank you!

To review the GMAT Club's Forums Posting Guidelines, please follow these links: Quantitative | Verbal Please note - we may remove posts that do not follow our posting guidelines. Thank you.

_________________

cheers
Its Now Or Never

Re: DS EXPONENTS   [#permalink] 11 Sep 2008, 18:34
Display posts from previous: Sort by