Register    Login    Search    Rss Feeds

Forums Home » Ask An Instructor » GMAT Math » Manhattan GMAT Math Strategy Guides


 Page 1 of 1 [ 3 posts ] 



  Print view Print view Previous topic Previous topic | Next topic Next topic
 

4th Edition,Math Strategy Guide3 Problemset Page no.187,Q9

Author Message
rahul_var25
 Post subject: 4th Edition,Math Strategy Guide3 Problemset Page no.187,Q9
 Post Posted: Thu Sep 15, 2011 6:20 am 
Offline
Students


Posts: 2
Is X3>x2 ? "x3 read as X cube and x2 as x square".

1. x>0
2. x2>x

Rephrasing : x3>x2
x3-x2>0
x2(x-1)>0 this means either both are +ve or both are -ve. Since x2 can never be -ve, therefore
(x-1)>0 or x>1 ?

from satement 1) X is positive but it cannot tell anything about x>1 b'coz it could 0<x<1 . Insufficient

from satement 2) X2>0 or X2-X>0 or x(x-1)>0
x>0 and x>1 from here we can infer that "x>1" . So the answer should be B . B'coz it seems sufficient.

But in guide answer is C.

CAN ANYONE EXPLAIN ME. THANK yOU.
Top 
JohnHarris
 Post subject: Re: 4th Edition,Math Strategy Guide3 Problemset Page no.187,Q9
 Post Posted: Thu Sep 15, 2011 1:34 pm 
Offline
Forum Guests


Posts: 38
Hi rahul,

A good approach but you stumbled a bit. Just as in the rephrasing of the question, statement two leads to two possibilities of the two factors which must both be examined, i.e.
from satement 2) X2>0 or X2-X>0 or x(x-1)>0
implies we have x and x-1 are either both positive or both negative. From statement 2 alone we do not know that x is greater than zero. So we must allow both of those possibilities. So both x and x-1 could be negative, say, for example, -1/10 and -11/10 in which case x^3 < x^2 and statement two alone is not sufficient.

When you put bot statement (1) and statement (2) together, you can then say, since x is greater than zero [from (1)], x must be greater than 1 [from (2)] and the two statements together are sufficient.
Top 
tim
 Post subject: Re: 4th Edition,Math Strategy Guide3 Problemset Page no.187,Q9
 Post Posted: Sat Sep 24, 2011 1:20 pm 
Offline
ManhattanGMAT Staff


Posts: 2183
Location: Southwest Airlines, seat 21C
Thanks John. One additional thing the original poster missed is that x^2 need not always be positive - you have to consider the possibility that x=0. Of course, statement 1 deals with this quite effectively once you combine the statements..

_________________
Tim Sanders
Manhattan GMAT Instructor
Top 
Display posts from previous:  Sort by  
 
 Page 1 of 1 [ 3 posts ] 





Who is online

Users browsing this forum: No registered users and 1 guest

 
 

 
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to: