Register    Login    Search    Rss Feeds

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


 Page 1 of 1 [ 6 posts ] 



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

Algebraic Equations

Author Message
afvatcha
 Post subject: Algebraic Equations
 Post Posted: Fri Nov 20, 2009 1:11 am 
Offline
Course Students


Posts: 34
Keep getting this problem wrong.

y(3x-5/2)=y

I can think of a number of different ways to solve this problem, but only one way is right. I don't know what makes that the right way.

It appears you should first divide both sides by y, so 3x-5/2=1. Then solve the rest. But why can't you first multiply by 2. So, 2y(3x-5)=2y...3x-5=1. This obviously changes the answer. Why is the first way correct?
Top 
nitin_prakash_khanna
 Post subject: Re: Algebraic Equations
 Post Posted: Sat Nov 21, 2009 10:45 am 
Offline
Students


Posts: 68
I suggest you post the whole problem (and the source).

If its DS question then, there could be multiple interpretation to the eq.
like...
y(3x-5/2)=y
could mean
y(3x-5/2)-y=0
y(3x-7/2)=0
so either y=0 or x=7/6 will satisfy the above.

so please post the complete problem.

One small comment on what you mentioned below. Even if you multiply it by 2 first.
and then divide by y. it will be 6x-5=2 not 3x-5=1 as you have written.
Top 
afvatcha
 Post subject: Re: Algebraic Equations
 Post Posted: Tue Nov 24, 2009 8:22 pm 
Offline
Course Students


Posts: 34
Hi,

That is the entire problem. It's not a ds prob.

Quick question about your response. Why would it be 6x-5=y. If I multiply both sides of the equation by two wouldn't the denominator cancel. ie.(3x-5)/2=y which is 3x-5=2y...(removed the one y from the problem for clarity sake).

But most of all thanks for your help.
Top 
pritesh.suvarna
 Post subject: Re: Algebraic Equations
 Post Posted: Sun Nov 29, 2009 9:57 pm 
Offline
Students


Posts: 8
If the problem is
y(3x-5/2)=y

3x-5/2 = 1 (hope the qst. means 5/2 and NOT (3x-5)/2 )
(6x-5)/2 = 1 (equalise denominator)
x = 7/6

Thanks
Pritesh
Top 
jeetdan
 Post subject: Re: Algebraic Equations
 Post Posted: Tue Dec 01, 2009 4:40 pm 
Offline


Posts: 3
y(x-5/2) = y

y(x-5/2) - y =0

y(x - 5/2-1) = 0

thus
either : y =0

or : x= 7/2

If we were given that y is positive integer we would have directly cancelled out y on both sides. But since we aren't told that y is positive we have to substract y on both sides and so on.
Top 
Ben Ku
 Post subject: Re: Algebraic Equations
 Post Posted: Thu Dec 24, 2009 6:36 pm 
Offline
ManhattanGMAT Staff


Posts: 823
jeetdan wrote:
y(x-5/2) = y

y(x-5/2) - y =0

y(x - 5/2-1) = 0

thus
either : y =0

or : x= 7/2

If we were given that y is positive integer we would have directly cancelled out y on both sides. But since we aren't told that y is positive we have to substract y on both sides and so on.


Jeetdan's response is correct. NEVER divide both sides by a variable unless you know it's not zero. The correct approach is to bring all terms to one side of the equation, then factor.

_________________
Ben Ku
Instructor
ManhattanGMAT
Top 
Display posts from previous:  Sort by  
 
 Page 1 of 1 [ 6 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: