|
Author |
Message |
|
Manager
Joined: 30 Oct 2010
Posts: 60
Followers: 2
Kudos [?]:
8
[0], given: 38
|
Bunuel wrote: charlemagne wrote: ilhom1986 wrote: From the given question I found that IF A>0 than C>0 IF A<0 than C<0 (I answered this question in less than a minute and choose B, because I didn't consider B to be equal to zero) Good question for GMAT starter, easy to get into trap...  I did this sometime back...bt I dont seem to have considered that B may be equal to 0, and still chose C.  CAn anyone show how B=0 case make a difference? Sure. For example if we put b\neq{0} in the stem then the answer would become B, instead of C. IF THE QUESTION WERE:If b\neq{0} is a^7*b^2*c^3>0 ? Since b\neq{0} then b^2>0, so we can reduce by it (b^2 does not affect given inequality at all) and the question becomes: is a^7*c^3>0? So, the question basically asks whether a and c have the same sign. (Notice that if b\neq{0} were not given then b^2\geq{0}, and we could not reduce by it) (1) bc<0. Not sufficient. (2) ac>0 --> a and c have the same sign. Sufficient. Answer: B. Hope it's clear. P.S. Solution to he original question is given here: m11-75085.html#p1073868Yes....much clearer....Thank you!!! Another kudos to you....  BTW, with so many of these already on your name, you must be going crazy with the amount of auto-notification emails coming your way??
_________________
KUDOS-ing does'nt cost you anything, but might just make someone's day!!!
"Musings of a Muddled Muggle Mind"
http://felinesmile.blogspot.in/
...comments Welcome 
|
|
|
|
|
|
|
|
|
GMAT Club team member
Joined: 02 Sep 2009
Posts: 11628
Followers: 1802
Kudos [?]:
9618
[1] , given: 829
|
1
This post received
KUDOS
charlemagne wrote: Yes....much clearer....Thank you!!! Another kudos to you.... BTW, with so many of these already on your name, you must be going crazy with the amount of auto-notification emails coming your way?? That's not a problem. Also one should consider kudos not only as a "thank you" to user whose post was helpful, but also as a tool to distinguish a valuable post. Notice that posts with more than 1 kudos have different color, also notice that total # of kudos is shown just beside the topic name, so by giving a kudos to a post you are drawing an attention of other users to the helpful material and thus are contributing to the community.
_________________
PLEASE READ AND FOLLOW: 11 Rules for Posting!!!
RESOURCES: [GMAT MATH BOOK]; 1. Triangles; 2. Polygons; 3. Coordinate Geometry; 4. Factorials; 5. Circles; 6. Number Theory
COLLECTION OF QUESTIONS:
PS: 1. Tough and Tricky questions; 2. Hard questions; 3. Hard questions part 2; 4. Standard deviation; 5. Tough Problem Solving Questions With Solutions; 6. Probability and Combinations Questions With Solutions; 7 Tough and tricky exponents and roots questions; 8 12 Easy Pieces (or not?); 9 Bakers' Dozen; 10 Algebra set. NEW!!!
DS: 1. DS tough questions; 2. DS tough questions part 2; 3. DS tough questions part 3; 4. DS Standard deviation; 5. Inequalities; 6. 700+ GMAT Data Sufficiency Questions With Explanations; 7 Tough and tricky exponents and roots questions; 8 The Discreet Charm of the DS ; 9 Devil's Dozen!!!; 10 Number Properties set. NEW!!!
What are GMAT Club Tests?
25 extra-hard Quant Tests
Find out what's new at GMAT Club - latest features and updates
|
|
|
|
|
|
Manager
Joined: 30 Oct 2010
Posts: 60
Followers: 2
Kudos [?]:
8
[0], given: 38
|
Bunuel wrote: charlemagne wrote: Yes....much clearer....Thank you!!! Another kudos to you.... BTW, with so many of these already on your name, you must be going crazy with the amount of auto-notification emails coming your way?? That's not a problem. Also one should consider kudos not only as a "thank you" to user whose post was helpful, but also as a tool to distinguish a valuable post. Notice that posts with more than 2 kudos have different color, also notice that total # of kudos is shown just beside the topic name, so by giving a kudos to a post you are drawing an attention of other users to the helpful material and thus are contributing to the community. Gee...didnt know that...thanks for the info bunuel! Quote: lso notice that total # of kudos is shown just beside the topic name, Didn't get this part though. 
_________________
KUDOS-ing does'nt cost you anything, but might just make someone's day!!!
"Musings of a Muddled Muggle Mind"
http://felinesmile.blogspot.in/
...comments Welcome
|
|
|
|
|
|
GMAT Club team member
Joined: 02 Sep 2009
Posts: 11628
Followers: 1802
Kudos [?]:
9618
[0], given: 829
|
|
|
|
|
|
|
Intern
Joined: 11 Jan 2010
Posts: 32
Followers: 1
Kudos [?]:
34
[0], given: 6
|
Here is how: Question: Is (A^7) * (B^2) * (C^3) > 0?
Statement 1: BC < 0
Evaluate the Y/N question:
Is (A^7) * (B*C)^2 * (C) > 0?
(B*C)^2 > 0
But, if A > 0 and C > 0 then expression > 0 => Yes
However, if A > 0 and C < 0 then expression < 0 => No
So, S1 is not sufficient. Eliminate A and D.
Statement 2: AC > 0
Evaluate the Y/N question:
Is expression (A^7) * (B^2) * (C^3) > 0?
Combine A and C.
Is (A*C)^3 * A^4 * B^2 > 0
S2 says A*C > 0 ; so (A*C)^3 > 0
A^4 is always positive even if A were negative; so A^4 > 0
B^2 is always positive even if B were negative; so B^2 > 0
So, the expression is always positive even if A and/or B were negative.
expression > 0
Only exception to this is: if B = 0, then expression = 0
S2 is NOT sufficient.
Take both S1 and S2.
This eliminate the possibility that B may be 0.
So, C is correct.
Last edited by vshrivastava on 23 Apr 2013, 06:59, edited 1 time in total.
|
|
|
|
|
|
GMAT Club team member
Joined: 02 Sep 2009
Posts: 11628
Followers: 1802
Kudos [?]:
9618
[1] , given: 829
|
1
This post received
KUDOS
vshrivastava wrote: I'm sorry if I'm missing something, but I think the right answer is B.
Here is how: Question: Is (A^7) * (B^2) * (C^3) > 0?
Statement 1: BC < 0
Evaluate the Y/N question:
Is (A^7) * (B*C)^2 * (C) > 0?
(B*C)^2 > 0
But, if A > 0 and C > 0 then expression > 0 => Yes
However, if A > 0 and C < 0 then expression < 0 => No
So, S1 is not sufficient. Eliminate A and D.
Statement 2: AC > 0
Evaluate the Y/N question:
Is (A^7) * (B^2) * (C^3) > 0?
Combine A and C.
Is (A*C)^3 * A^4 * B^2 > 0
S2 says A*C > 0 ; so (A*C)^3 > 0
A^4 is always positive even if A were negative; so A^4 > 0
B^2 is always positive even if B were negative; so B^2 > 0
So, the expression is always positive even if A and/or B were negative.
S2 is sufficient.
B is correct.
What do you guys think? Notice that when we consider the second statement we know nothing about the value of b, so if b=0, then a^7*b^2*c^3=0 ( a^7*b^2*c^3 is NOT greater than 0). That's why (2) is not sufficient. Similar questions to practice: is-x-2-y-5-z-0-1-xz-y-0-2-y-z-98341.htmlis-x-7-y-2-z-3-0-1-yz-0-2-xz-127692.htmlm21-q30-96613.htmlHope it helps.
_________________
PLEASE READ AND FOLLOW: 11 Rules for Posting!!!
RESOURCES: [GMAT MATH BOOK]; 1. Triangles; 2. Polygons; 3. Coordinate Geometry; 4. Factorials; 5. Circles; 6. Number Theory
COLLECTION OF QUESTIONS:
PS: 1. Tough and Tricky questions; 2. Hard questions; 3. Hard questions part 2; 4. Standard deviation; 5. Tough Problem Solving Questions With Solutions; 6. Probability and Combinations Questions With Solutions; 7 Tough and tricky exponents and roots questions; 8 12 Easy Pieces (or not?); 9 Bakers' Dozen; 10 Algebra set. NEW!!!
DS: 1. DS tough questions; 2. DS tough questions part 2; 3. DS tough questions part 3; 4. DS Standard deviation; 5. Inequalities; 6. 700+ GMAT Data Sufficiency Questions With Explanations; 7 Tough and tricky exponents and roots questions; 8 The Discreet Charm of the DS ; 9 Devil's Dozen!!!; 10 Number Properties set. NEW!!!
What are GMAT Club Tests?
25 extra-hard Quant Tests
Find out what's new at GMAT Club - latest features and updates
|
|
|
|
|
|
Intern
Joined: 04 Apr 2013
Posts: 9
Concentration: Finance, Economics
Followers: 0
Kudos [?]:
0
[0], given: 0
|
Answer C.
Rule out A, B and D immediately as both will be required.
So there are two outcomes based on (1) and (2):
Outcome 1: A = +ve, B = -ve, C = +ve
Gives: (+ve)^7 x (-ve)^2 x (+ve)^3 = (+ve) x (+ve) x (+ve) = +ve
OR
Outcome 2: A = -ve, B = +ve, C = -ve (i.e. opposite of outcome 1)
Gives: (-ve)^7 x (+ve)^2 x (-ve)^3 = (-ve) x (+ve) x (-ve) = +ve
So answer is C, as both possible options give the answer i.e. A^7 x B^2 x C^3 > 0
BUT TOOK ME 4:00mins TO GET THERE!!!
|
|
|
|
|
|
Senior Manager
Joined: 16 Jun 2012
Posts: 357
Followers: 2
Kudos [?]:
77
[0], given: 71
|
topmbaseeker wrote: Is A^7 B^2 C^3 > 0 ? 1. BC < 0 2. AC > 0 Source: GMAT Club Tests - hardest GMAT questions I did the question too quick to fall in the trap. Very good question!. C is correct. A^7*B^2*C^3 = (A^6*B^2*C^2)*(C*A) If B = 0, (A^6*B^2*C^2)*(C*A) = 0. If B #0, the sign of (A^6*B^2*C^2)*(C*A) depends on (C*A) We must combine statement 1 and 2 to conclude: B #0 and (C*A) > 0 ==> (A^6*B^2*C^2)*(C*A) > 0.
_________________
Please +1 KUDO if my post helps. Thank you.
Choosing correct answers is not as important as explaining why other answers are wrong. Keep learning everyday!
|
|
|
|
|
|
Intern
Joined: 15 Sep 2012
Posts: 1
Followers: 0
Kudos [?]:
0
[0], given: 1
|
[quote="topmbaseeker"]Is A^7 B^2 C^3 > 0 ?
1. BC < 0
2. AC > 0
for AC>0
either both are negative or positive
we dont have depend on value of B,bcause square of anything is positive
in this case if A and c ARE NEGATIVE THEN 2 NEGATIVE SIGNS LEFT CANCEL OUT EACH OTHER AND SQUARE OF B IS POSITIVE,SO ANSWER IS POSITIVE
AND IF A AND C ARE POSITIVE AGAIN SQUARE OF B IS POSITIVE
so answer is B
|
|
|
|
|
|
GMAT Club team member
Joined: 02 Sep 2009
Posts: 11628
Followers: 1802
Kudos [?]:
9618
[0], given: 829
|
|
|
|
|
|
|
Intern
Joined: 03 May 2013
Posts: 1
Followers: 0
Kudos [?]:
0
[0], given: 1
|
I think the answer is B, S2 alone is sufficient.
A^7, B^2, C^3 can be considered individually.
S1 cannot account for A at all, and A^7 could be either positive or negative so S1 is out.
Looking at S2, B^2 is automatically positive, so we have only A^7 and C^3 to worry about. Since AC>0, then (AC)^3>0 so we have only A^4 to worry about, but A^4 is positive by definition.
So S2 alone is sufficient. S1 doesn't work at all.
|
|
|
|
|
|
GMAT Club team member
Joined: 02 Sep 2009
Posts: 11628
Followers: 1802
Kudos [?]:
9618
[0], given: 829
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
close
