Register    Login    Search    Rss Feeds

Forums Home » Ask An Instructor » GMAT Math » Manhattan GMAT CAT Math


 Page 1 of 1 [ 7 posts ] 



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

work your abs

Author Message
trided
 Post subject: work your abs
 Post Posted: Thu Nov 05, 2009 3:42 am 
Offline


Posts: 3
hi!
I have a question regarding this one

What is the value of y?

(1) 3|x2 – 4| = y – 2

(2) |3 – y| = 11

I met this kind of questions before and thought the solution could be like this one
l3-yl=11
y=-8 or y = 14
MGMAT learned me that in order to solve this one you should just try each number in the initial equation:
3 - -8= 11 OK
3-14=11 not ok

so B
but the correct answer here is C

what do I miss?

thanks!!!!
Top 
RonPurewal
 Post subject: Re: work your abs
 Post Posted: Mon Nov 09, 2009 7:40 pm 
Offline
ManhattanGMAT Staff


Posts: 7146
Quote:
I met this kind of questions before and thought the solution could be like this one
l3-yl=11
y=-8 or y = 14
MGMAT learned me that in order to solve this one you should just try each number in the initial equation:
3 - -8= 11 OK
3-14=11 not ok


hi -

yes, you should try these solutions in the original equation. (if the right side is just a NUMBER, as it is here, then you technically don't have to. however, if you don't know EXACTLY when you should and shouldn't check, then you should always check, just to be sure.)

the ORIGINAL equation contains an absolute value. do not neglect this.
i.e., you appear to be testing the equation
3 - y = 11
when the original equation is in fact
|3 - y| = 11

if we plug into the true original equation, -8 gives |11| = 11, and 14 gives |-11| = 11. both are true.
Top 
imanemekouar
 Post subject: Re: work your abs
 Post Posted: Thu Jan 28, 2010 3:37 pm 
Offline
Students


Posts: 24
Hi ron, I have a big question for you.When should a check the result that from an absolute value equation in this equation?And if I did What 's the conclusion?
Top 
RonPurewal
 Post subject: Re: work your abs
 Post Posted: Fri Feb 19, 2010 8:22 am 
Offline
ManhattanGMAT Staff


Posts: 7146
imanemekouar wrote:
Hi ron, I have a big question for you.When should a check the result that from an absolute value equation in this equation?


it's not really worth memorizing complex rules as to when to check and when not to check.
instead, just check the solutions ANY time you have an absolute value equation.

it only takes a few seconds to plug into an equation and check, so, even if you are checking when you don't have to, it's no big loss.

Quote:
And if I did What 's the conclusion?


if you plug the number in and it works, it's a solution.

if you plug a number in and it doesn't work, it's not a solution.
Top 
harishamid
 Post subject: Re: work your abs
 Post Posted: Wed Oct 27, 2010 11:44 am 
Offline
Students


Posts: 2
Ron,

I thought we get two equations from Statement 1.

3|x^2 – 4| = y – 2
=> x^2 - 4 = (y - 2)/3 and -(x^2 - 4) = (y - 2)/3

solving these two equations

=> y = 2.

What I'm doing wrong in this?

Thanks
Top 
harishamid
 Post subject: Re: work your abs
 Post Posted: Wed Oct 27, 2010 2:11 pm 
Offline
Students


Posts: 2
Sorry my mistake. I know what I did wrong.
Top 
mschwrtz
 Post subject: Re: work your abs
 Post Posted: Wed Oct 27, 2010 5:39 pm 
Offline
ManhattanGMAT Staff


Posts: 506
Cool. Easiest post I've had to answer today.
Top 
Display posts from previous:  Sort by  
 
 Page 1 of 1 [ 7 posts ] 





Who is online

Users browsing this forum: No registered users and 0 guests

 
 

 
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: