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pradeepkaushal9518: Legendary Member; Posts: 1309; Joined: Wed Mar 17, 2010 11:41 pmFollowed by: 5 members; Thanked: 33 times

by pradeepkaushal9518 » Mon Aug 30, 2010 4:50 am

is x^2 equal to xy?

1.x^2-y^2=(x+5)(y-5)

2.x=y

A SMALL TOWN GUY

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Source: Beat The GMAT — Data Sufficiency | Mon Aug 30, 2010 4:50 am

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Rahul@gurome: GMAT Instructor; Posts: 1179; Joined: Sun Apr 11, 2010 9:07 pmFollowed by: 88 members; Location: Milpitas, CA; Thanked: 447 times

by Rahul@gurome » Mon Aug 30, 2010 6:25 am

pradeepkaushal9518 wrote:is x^2 equal to xy?

1.x^2-y^2=(x+5)(y-5)

2.x=y

(1) x^2 - y^2 = xy + 5y - 5x - 25
x^2 + 5x + 25 = y^2 + 5y + xy
But this does not give any info on whether x^2 equal to xy.
So, (1) is NOT SUFFICIENT.

(2) x= y implies x^2 = xy
So, (2) is SUFFICIENT.

The correct answer is [spoiler](B)[/spoiler].

Rahul Lakhani
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Gurpinder: Legendary Member; Posts: 659; Joined: Mon Dec 14, 2009 8:12 amFollowed by: 3 members; Thanked: 32 times

by Gurpinder » Mon Aug 30, 2010 9:37 am

Hey Rahul,

Can you tell me what I am doing wrong?

The way I thought of it, statement 1 is sufficient.

(1) x^2-y^2=(x+5)(y-5)

Can't we use this to get the values for X and Y?

X= -5 and Y = 5

Therefore x^2=25 and (5)(-5)=-25

So Stmt 1 is sufficient?

Please advise,

"Do not confuse motion and progress. A rocking horse keeps moving but does not make any progress."
- Alfred A. Montapert, Philosopher.

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puneetdua: Senior | Next Rank: 100 Posts; Posts: 87; Joined: Wed Jun 16, 2010 7:57 pm; Location: Delhi,India; Thanked: 1 times

by puneetdua » Mon Aug 30, 2010 10:17 am

Hi,

In my first attemp i also thought answer is B but after reading Guprinder's comments - i lost the confidence in option B ...

i tried putting several values in option 1 to satisfy the relation but only -5 / 5 are satisgying - so D might be the answer ...

curious to know the OA..Pradeep can you please post the answer please..

Thanks
Puneet

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reply2spg: Legendary Member; Posts: 1261; Joined: Sun Sep 14, 2008 3:46 am; Thanked: 27 times; GMAT Score:570

by reply2spg » Mon Aug 30, 2010 11:54 am

Answer should be B. Here's how I got it,

A. x^2-y^2=(x+5)(y-5)

which means
x^2-y^2 = x^2 - 25

get rid of x^2 from both sides. you will get

y^2 = 25 and y = 5 or -5. You don't know x. so you can not say x^2=xy.

Gurpinder wrote:Hey Rahul,

Can you tell me what I am doing wrong?

The way I thought of it, statement 1 is sufficient.

(1) x^2-y^2=(x+5)(y-5)

Can't we use this to get the values for X and Y?

X= -5 and Y = 5

Therefore x^2=25 and (5)(-5)=-25

So Stmt 1 is sufficient?

Please advise,

Sudhanshu
(have lot of things to learn from all of you)

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Stuart@KaplanGMAT: GMAT Instructor; Posts: 3225; Joined: Tue Jan 08, 2008 2:40 pmFollowed by: 614 members; Location: Toronto; Thanked: 1710 times; GMAT Score:800

by Stuart@KaplanGMAT » Mon Aug 30, 2010 1:04 pm

Gurpinder wrote:Hey Rahul,

Can you tell me what I am doing wrong?

The way I thought of it, statement 1 is sufficient.

(1) x^2-y^2=(x+5)(y-5)

Can't we use this to get the values for X and Y?

X= -5 and Y = 5

Therefore x^2=25 and (5)(-5)=-25

So Stmt 1 is sufficient?

Please advise,

Hi,

x=5 and y=5 also makes the statement correct:

25 - 25 = (10)(0)

So, based on (1), the values could be:

x=-5 y=5; does (-5)^2 = (-5)(5)? NO

x=5 y=5; does (5)^2 = (5)(5)? YES

Stuart Kovinsky | Kaplan GMAT Faculty | Toronto

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