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

Expert replies

melguy: Master | Next Rank: 500 Posts; Posts: 335; Joined: Mon Mar 21, 2011 11:31 pmFollowed by: 2 members; Location: Australia / India; Thanked: 37 times

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

Like this post

Source: Beat The GMAT — Data Sufficiency | Fri Aug 30, 2013 10:29 pm

GMAT/MBA Expert

[email protected]: Elite Legendary Member; Posts: 10392; Joined: Sun Jun 23, 2013 6:38 pmFollowed by: 511 members; Location: Palo Alto, CA; Thanked: 2867 times; 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]

Like this post

melguy: Master | Next Rank: 500 Posts; Posts: 335; Joined: Mon Mar 21, 2011 11:31 pmFollowed by: 2 members; Location: Australia / India; Thanked: 37 times

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!

Like this post

GMAT/MBA Expert

[email protected]: Elite Legendary Member; Posts: 10392; Joined: Sun Jun 23, 2013 6:38 pmFollowed by: 511 members; Location: Palo Alto, CA; Thanked: 2867 times; 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]

Like this post

faraz_jeddah: Master | Next Rank: 500 Posts; Posts: 358; Joined: Thu Apr 18, 2013 9:46 amFollowed by: 7 members; Location: Jeddah, Saudi Arabia; Thanked: 42 times; 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.

Like this post

GMAT/MBA Expert

Brent@GMATPrepNow: GMAT Instructor; Posts: 16207; Joined: Mon Dec 08, 2008 6:26 pmFollowed by: 1268 members; Location: Vancouver, BC; Thanked: 5254 times; 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

Like this post

Mission2012: Master | Next Rank: 500 Posts; Posts: 184; Joined: Sun Aug 19, 2012 10:04 pmFollowed by: 2 members; Thanked: 10 times

by Mission2012 » Tue Sep 03, 2013 4:49 am

Is OA A

If you find my post useful -> please click on "Thanks"

Like this post

faraz_jeddah: Master | Next Rank: 500 Posts; Posts: 358; Joined: Thu Apr 18, 2013 9:46 amFollowed by: 7 members; Location: Jeddah, Saudi Arabia; Thanked: 42 times; 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.

Like this post

Java_85: 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

Like this post

melguy: Master | Next Rank: 500 Posts; Posts: 335; Joined: Mon Mar 21, 2011 11:31 pmFollowed by: 2 members; Location: Australia / India; Thanked: 37 times

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.

Like this post

Post new topic Post Reply

• Page 1 of 1

GMAT Course Reviews

559 reviews

1168 reviews

431 reviews

359 reviews

147 reviews

5/5

5 Star (560 Reviews)

"Target Test Prep is the closest to the official version of the GMAT exam, about 99% accuracy in terms of the quality and quantity of information. The course has excellently created singular sets of focused lessons and tests for every possible topic that one could come across in the official GMAT exam."

"The TTP course maximizes the efficiency of the time you spend studying. It will take time and effort but I could almost guarantee that if you complete the course exactly as it is laid out you will get an amazing score. They also have a very responsive team willing to help with any questions you might have."

"TTP has two things that I think no other test prep company offers: A teaching approach that reinforces understanding and an attitude that will give you the mental preparedness needed to succeed on the test. TTP gives you a deep understanding of the concept you need to know while teaching you how to think."

Write a Review