BREAKING: Target Test Prep releases Brand New 2026 On Demand GMAT prep course

Redeem

In terms of y, x =

This topic has expert replies
Post new topic Post Reply
Previous Topic Next Topic
Master | Next Rank: 500 Posts
Posts: 435
Joined: Wed Nov 16, 2011 7:27 am
Thanked: 48 times
Followed by:16 members

In terms of y, x =

by alex.gellatly » Sun Aug 26, 2012 12:25 am
If x>=0 and x = √(8xy-16y^2), then, in terms of y, x =

-4y
y/4
y
4y
4y^2

Thanks
Sorry about the missing part, it has been changed!
Last edited by alex.gellatly on Sun Aug 26, 2012 7:41 pm, edited 1 time in total.
A useful website I found that has every quant OG video explanation:

https://www.beatthegmat.com/useful-websi ... tml#475231
Top
Source: — Problem Solving |
User avatar
Community Manager
Posts: 1060
Joined: Fri May 13, 2011 6:46 am
Location: Utrecht, The Netherlands
Thanked: 318 times
Followed by:52 members

by neelgandham » Sun Aug 26, 2012 1:56 am
Can you post the complete question please?
'If x >= MISSING and x = sqrt(8xy-16y^2) , then, in terms of y, x = '
Anil Gandham
Welcome to BEATtheGMAT | Photography | Getting Started | BTG Community rules | MBA Watch
Check out GMAT Prep Now's online course at https://www.gmatprepnow.com/
Top

GMAT/MBA Expert

User avatar
GMAT Instructor
Posts: 16207
Joined: Mon Dec 08, 2008 6:26 pm
Location: Vancouver, BC
Thanked: 5254 times
Followed by:1268 members
GMAT Score:770

by Brent@GMATPrepNow » Sun Aug 26, 2012 6:16 am
alex.gellatly wrote:If x>0 and x = √(8xy-16y^2), then, in terms of y, x =

-4y
y/4
y
4y
4y^2

Thanks
Edited.

Note: I'm assuming that the missing portion (above) tells us that x is greater than or equal to 0.

Given: x = sqrt(8xy-16y^2)
Square both sides: x^2 = 8xy-16y^2
Set equation equal to zero: x^2 - 8xy + 16y^2 = 0
Factor left side: (x - 4y)(x - 4y) = 0
This tells us that x - 4y = 0, which means x = 4y

Answer = D

Cheers,
Brent
Last edited by Brent@GMATPrepNow on Sun Aug 26, 2012 5:45 pm, edited 4 times in total.
Brent Hanneson - Creator of GMATPrepNow.com
Image
Top
Master | Next Rank: 500 Posts
Posts: 265
Joined: Tue Jul 03, 2012 11:04 pm
Followed by:6 members

by grandh01 » Sun Aug 26, 2012 4:52 pm
x^2 - 8xy + 16y^2 = 0
(x-4y)(x-4y)=0

x-4y=0
x=4y

Is the answer not D?
Top
Master | Next Rank: 500 Posts
Posts: 141
Joined: Tue Oct 04, 2011 5:17 am
Thanked: 25 times

by coolhabhi » Mon Aug 27, 2012 12:58 pm
alex.gellatly wrote:If x>0 and x = √(8xy-16y^2), then, in terms of y, x =

-4y
y/4
y
4y
4y^2

Thanks
Do reverse engineering. Put the options in the equation "x = √(8xy-16y^2)".
But choosing the right options to substitute is also necessary.

Since the equation has a square root, (8xy-16y^2) should turn out to be a square number. If you observe option D, suits our requirement.
Just try it.
x = √(8xy-16y^2)
substituting x = 4y.
RHS = √(8(4y)y-16y^2)
RHS = √(32y^2-16y^2)
RHS = √(16y^2)
RHS = +4y or -4y.
Since x > 0 we can say x = 4y.
[spoiler]Answer: D[/spoiler]
Top
Master | Next Rank: 500 Posts
Posts: 435
Joined: Wed Nov 16, 2011 7:27 am
Thanked: 48 times
Followed by:16 members

by alex.gellatly » Wed Aug 29, 2012 8:46 pm
Brent@GMATPrepNow wrote: Set equation equal to zero: x^2 - 8xy + 16y^2 = 0
Factor left side: (x - 4y)(x - 4y) = 0
This tells us that x - 4y = 0, which means x = 4y
Thank you for your post but I'm still a little unclear with your factorization. If you add -4x and -4x you get -8x... but we need it to equal -8xy, do we not?... but here we have no y.... Am I missing something/

Thaks
A useful website I found that has every quant OG video explanation:

https://www.beatthegmat.com/useful-websi ... tml#475231
Top
User avatar
Master | Next Rank: 500 Posts
Posts: 167
Joined: Fri Mar 09, 2012 8:35 pm
Thanked: 39 times
Followed by:3 members

by adthedaddy » Thu Aug 30, 2012 12:04 am
Thank you for your post but I'm still a little unclear with your factorization. If you add -4x and -4x you get -8x... but we need it to equal -8xy, do we not?... but here we have no y.... Am I missing something/


Dear Alex,

The factorization goes this way -

x^2-8xy+16y^2 = 0
x^2-4xy-4xy+16y^2=0
x(x-4y)-4y(x-4y)=0
(x-4y)(x-4y)=0
i.e. (x-4y)^2=0
i.e. x-4y=0 => x=4y

OR

If you can identify that x^2-8xy+16y^2 = 0 is an expansion of (x-4y)^2 then you can direcly factorize the term.

Hope this satisfies.
"Your time is limited, so don't waste it living someone else's life. Don't be trapped by dogma - which is living with the results of other people's thinking. Don't let the noise of others' opinions drown out your own inner voice. And most important, have the courage to follow your heart and intuition. They somehow already know what you truly want to become. Everything else is secondary" - Steve Jobs
Top
Post new topic Post Reply
• Page 1 of 1