Register    Login    Search    Rss Feeds

Forums Home » Ask An Instructor » GMAT Math » GMAT Prep Math


 Page 1 of 1 [ 3 posts ] 



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

Is 2x-3y < x^2?

Author Message
ambikasrinivas
 Post subject: Is 2x-3y < x^2?
 Post Posted: Sun Jun 05, 2011 8:25 pm 
Offline
Course Students


Posts: 13
From GMAT Prep exam

Is 2x-3y < x^2?

(1) 2x-3y = -2
(2) x>2 and y>0

Answer D

I don’t understand how to get this answer with the statements alone. I can see how to use them together, but not alone. Thanks!
Top 
george.kourdin
 Post subject: Re: Is 2x-3y < x^2?
 Post Posted: Sun Jun 05, 2011 11:18 pm 
Offline
Course Students


Posts: 98
this is what id do - if anyone has another way or see any mistakes- please reply

1) try to simplify the question stem. taking square root of both sides doesn't really get us anywhere. move on to answer choices

a) 2x-3y=-2
2x = 3y-2
plug it into the original we get 3y-2-3y<x^2
-2<x^2
sufficient ....x^2 will always be > 0 regardless of x so its AD

b) x>2 and y>0
so x is some number > 2 and y is positive
try plugging any combination of numbers x and y and x^2 will always be greater because -3 will always diminish the expression
Top 
RonPurewal
 Post subject: Re: Is 2x-3y < x^2?
 Post Posted: Tue Jun 07, 2011 8:12 am 
Offline
ManhattanGMAT Staff


Posts: 7146
george.kourdin wrote:
this is what id do - if anyone has another way or see any mistakes- please reply

1) try to simplify the question stem. taking square root of both sides doesn't really get us anywhere. move on to answer choices

a) 2x-3y=-2
2x = 3y-2
plug it into the original we get 3y-2-3y<x^2
-2<x^2
sufficient ....x^2 will always be > 0 regardless of x so its AD


i suppose this correct, but why are you messing with the equation?
the left-hand side of this choice is 2x - 3y. the left-hand side of the prompt question is *also* 2x - 3y. so, for heaven's sake, don't break up that expression.

this answer choice just tells you that 2x - 3y = -2. this can be substituted directly into the prompt question, without any algebra or rearrangement, to give "Is -2 < x^2?"
the answer is yes, because x^2 must be either 0 or positive.

Quote:
b) x>2 and y>0
so x is some number > 2 and y is positive
try plugging any combination of numbers x and y and x^2 will always be greater because -3 will always diminish the expression


you can also note that, if x > 2, then 2x is already less than x^2 (because 2x is 2 times x, and x^2 is x times x; we know that x > 2, so x times x > 2 times x.)
therefore, if 2x is already less than x^2, then 2x minus some positive number (i.e., 2x - 3y) -- which is an even smaller number -- will also be less than x^2.
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: