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

Redeem

Is x^2+y^2>3z?

This topic has expert replies
Post new topic Post Reply
Previous Topic Next Topic
User avatar
Master | Next Rank: 500 Posts
Posts: 335
Joined: Mon Mar 21, 2011 11:31 pm
Location: Australia / India
Thanked: 37 times
Followed by:2 members

Is x^2+y^2>3z?

by melguy » Fri Aug 30, 2013 10:29 pm
Hello

The approach I took for this problem

Statement 1

There are 3 variables and 2 equations. Hence we should be able to find the value of x and y.

So it should be sufficient to answer the Q.

It does not matches with the OA. Please help me with the problem. Thanks
Attachments
DS.JPG
Top
Source: — Data Sufficiency |

GMAT/MBA Expert

User avatar
Elite Legendary Member
Posts: 10392
Joined: Sun Jun 23, 2013 6:38 pm
Location: Palo Alto, CA
Thanked: 2867 times
Followed by:511 members
GMAT Score:800

by [email protected] » Fri Aug 30, 2013 11:28 pm
Hi melguy,

I'm going to give you a hint, then have you try this question again.

First, this is a YES/NO question; it's asking if x^2 + y^2 > 3z? It doesn't tell you ANYTHING about the variables. Second, is it possible that some (or all) of these variables = 0?

GMAT assassins aren't born, they're made,
Rich
Contact Rich at [email protected]
Image
Top
User avatar
Master | Next Rank: 500 Posts
Posts: 335
Joined: Mon Mar 21, 2011 11:31 pm
Location: Australia / India
Thanked: 37 times
Followed by:2 members

by melguy » Sat Aug 31, 2013 12:33 am
[email protected] wrote: some (or all) of these variables = 0?
It made the problem so easy! I broke down all the equations, testing with under roots and everything to see where the value of x^2 and y^2 is hiding but that surely is not the best method in this problem.

Thanks a lot Rich!
Top

GMAT/MBA Expert

User avatar
Elite Legendary Member
Posts: 10392
Joined: Sun Jun 23, 2013 6:38 pm
Location: Palo Alto, CA
Thanked: 2867 times
Followed by:511 members
GMAT Score:800

by [email protected] » Sat Aug 31, 2013 1:47 pm
Hi melguy,

There are ultimately 2 measures of "success" when choosing to answer a GMAT question:

1) Did you get the question correct?
2) How long did it take to do so?

If counting on your fingers is the fastest way that you can come up with to answer a question, then count on your fingers! Most GMAT questions have several potential approaches to solving, so flexibility on your part is a must (especially if you want to develop strong pacing skills). With what you've just learned, was the question made faster and easier to solve? Then remember these ideas the next time you face a similar question.

GMAT assassins aren't born, they're made,
Rich
Contact Rich at [email protected]
Image
Top
User avatar
Master | Next Rank: 500 Posts
Posts: 358
Joined: Thu Apr 18, 2013 9:46 am
Location: Jeddah, Saudi Arabia
Thanked: 42 times
Followed by:7 members
GMAT Score:730

by faraz_jeddah » Sun Sep 01, 2013 6:26 am
melguy wrote:Hello

Statement 1

There are 3 variables and 2 equations. Hence we should be able to find the value of x and y.

So it should be sufficient to answer the Q.
Just to add on a more general area - Ideally you need to have X number of distinct equations to solve for X variables.

Statement 1 is clearly not sufficient as it has only 2 variables.

I think you took the question stem as an equation. :(
A good question also deserves a Thanks.

Messenger Boy: The Thesselonian you're fighting... he's the biggest man i've ever seen. I wouldn't want to fight him.
Achilles: That's why no-one will remember your name.
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 Sep 01, 2013 7:15 am
melguy wrote:
There are 3 variables and 2 equations. Hence we should be able to find the value of x and y.
I should point out that the question isn't asking us to find the individual values of x, y and z. So, the rule about needing K different equations to solve a system with K variables doesn't apply here. Also, the question features inequalities, which also negates the rule.

For example, let's leave the target question as is (Is x^2 + y^2 > 3z?)
If statement 2 were "z < 0," then we'd have sufficient information to answer the target question. Here's why:
If z < 0, then 3z is negative
x^2 + y^2 will always be greater than or equal to zero.
So, it must be true that x^2 + y^2 > 3z

Cheers,
Brent
Brent Hanneson - Creator of GMATPrepNow.com
Image
Top
User avatar
Master | Next Rank: 500 Posts
Posts: 184
Joined: Sun Aug 19, 2012 10:04 pm
Thanked: 10 times
Followed by:2 members

by Mission2012 » Tue Sep 03, 2013 4:49 am
Is OA A
If you find my post useful -> please click on "Thanks"
Top
User avatar
Master | Next Rank: 500 Posts
Posts: 358
Joined: Thu Apr 18, 2013 9:46 am
Location: Jeddah, Saudi Arabia
Thanked: 42 times
Followed by:7 members
GMAT Score:730

by faraz_jeddah » Tue Sep 03, 2013 6:36 am
Mission2012 wrote:Is OA A
OA should be C

Statement 1 cannot be sufficient.

Statement 1 is true when x = y = z = 0 which would not satisfy the question stem.
Statement 1 is true when x = 2 ; y = 0 and z = 1/9 which would satisfy the question stem.
A good question also deserves a Thanks.

Messenger Boy: The Thesselonian you're fighting... he's the biggest man i've ever seen. I wouldn't want to fight him.
Achilles: That's why no-one will remember your name.
Top
User avatar
Master | Next Rank: 500 Posts
Posts: 234
Joined: Tue Jul 16, 2013 9:00 am
Location: West Virginia
Thanked: 9 times

by Java_85 » Tue Sep 03, 2013 9:15 pm
IMO A is the answer,

(1) We can calculate xy=2z and x^2+y^2=5z ==> Z>0 ==> 5z>3z ==> A or D
(2) We can not say, because x^2+y^2 might be 0 and 0<0 is not valid! ==>A
Top
User avatar
Master | Next Rank: 500 Posts
Posts: 335
Joined: Mon Mar 21, 2011 11:31 pm
Location: Australia / India
Thanked: 37 times
Followed by:2 members

by melguy » Tue Sep 03, 2013 9:23 pm
Hi Java

The OA is C. Try and solve the problem with the hint Rich has provided. It makes the problem lot easier to solve.
Top
Post new topic Post Reply
• Page 1 of 1