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

Redeem

Algebra

This topic has expert replies
Post new topic Post Reply
Previous Topic Next Topic
Source: — Data Sufficiency |
Senior | Next Rank: 100 Posts
Posts: 44
Joined: Sun Jun 28, 2009 8:58 am
Location: Online
Thanked: 13 times
GMAT Score:51

by GMATQuantCoach » Mon Jul 06, 2009 7:24 pm
Are you looking for (x-y)^2 or (x+y)^2?
FREE Weekly Online Office Hours

Upcoming FREE Online Class: Effective Calculations - 11/15/09 Sunday

Instructor | GMATPrepMath.com | GMAT Prep Courses Starting at $60 with FREE GMAT Math Formula Sheet

Choose from Topics:
Number Properties - Advanced Counting (Combinatorics) - Probability - Advanced Algebra in DS - Geometry - Word Problems - Fundamental Algebra in PS - Effective Calculations
Top

GMAT/MBA Expert

User avatar
GMAT Instructor
Posts: 2623
Joined: Mon Jun 02, 2008 3:17 am
Location: Montreal
Thanked: 1090 times
Followed by:355 members
GMAT Score:780

Re: Algebra

by Ian Stewart » Mon Jul 06, 2009 7:54 pm
Thonk02 wrote:Whatt is value of: (X+Y) ^2
_________
(X-Y)^2

1. X^2 + Y^2 = 3XY
2. XY = 3


Any ideas?

answer is D.
I expect the question means to ask for the value of:

(x+y)^2
(x-y)^2

and if so, the answer certainly is not D.

If you expand the top and bottom, the expression is equal to:

x^2 + 2xy + y^2
x^2 - 2xy + y^2

And using Statement 1, we can replace x^2 + y^2 on the top and bottom to get:

3xy + 2xy
3xy - 2xy

= 5/1 = 5

Using Statement 2, we won't get the necessary cancellation, so the Statement is insufficient. To be sure, you can plug in any pair of values which satisfy xy = 3; if x = 3 and y = 1, the expression is equal to 4, whereas if x = 2 and y = 3/2, the expression is equal to 49.

So unless I've misunderstood the typography in the question, the answer should be A.
For online GMAT math tutoring, or to buy my higher-level Quant books and problem sets, contact me at ianstewartgmat at gmail.com

ianstewartgmat.com
Top
Legendary Member
Posts: 752
Joined: Sun May 17, 2009 11:04 pm
Location: Tokyo
Thanked: 81 times
GMAT Score:680

Re: Algebra

by tohellandback » Mon Jul 06, 2009 8:45 pm
Ian Stewart wrote:
Thonk02 wrote:Whatt is value of: (X+Y) ^2
_________
(X-Y)^2

1. X^2 + Y^2 = 3XY
2. XY = 3


Any ideas?

answer is D.
I expect the question means to ask for the value of:

(x+y)^2
(x-y)^2

and if so, the answer certainly is not D.

If you expand the top and bottom, the expression is equal to:

x^2 + 2xy + y^2
x^2 - 2xy + y^2

And using Statement 1, we can replace x^2 + y^2 on the top and bottom to get:

3xy + 2xy
3xy - 2xy

= 5/1 = 5

Using Statement 2, we won't get the necessary cancellation, so the Statement is insufficient. To be sure, you can plug in any pair of values which satisfy xy = 3; if x = 3 and y = 1, the expression is equal to 4, whereas if x = 2 and y = 3/2, the expression is equal to 49.

So unless I've misunderstood the typography in the question, the answer should be A.
or Thonk has left out "X and Y are positive integers."
The powers of two are bloody impolite!!
Top

GMAT/MBA Expert

User avatar
GMAT Instructor
Posts: 2623
Joined: Mon Jun 02, 2008 3:17 am
Location: Montreal
Thanked: 1090 times
Followed by:355 members
GMAT Score:780

Re: Algebra

by Ian Stewart » Tue Jul 07, 2009 4:18 am
tohellandback wrote: or Thonk has left out "X and Y are positive integers."
If it's a real GMAT question, x and y can't possibly be positive integers - if they were, the two statements would be inconsistent.
For online GMAT math tutoring, or to buy my higher-level Quant books and problem sets, contact me at ianstewartgmat at gmail.com

ianstewartgmat.com
Top
Post new topic Post Reply
• Page 1 of 1