Thank you for using the timer - this advanced tool can estimate your performance and suggest more practice questions. We have subscribed you to Daily Prep Questions via email.

Customized for You

we will pick new questions that match your level based on your Timer History

Track Your Progress

every week, we’ll send you an estimated GMAT score based on your performance

Practice Pays

we will pick new questions that match your level based on your Timer History

Not interested in getting valuable practice questions and articles delivered to your email? No problem, unsubscribe here.

It appears that you are browsing the GMAT Club forum unregistered!

Signing up is free, quick, and confidential.
Join other 350,000 members and get the full benefits of GMAT Club

Registration gives you:

Tests

Take 11 tests and quizzes from GMAT Club and leading GMAT prep companies such as Manhattan GMAT,
Knewton, and others. All are free for GMAT Club members.

Applicant Stats

View detailed applicant stats such as GPA, GMAT score, work experience, location, application
status, and more

Books/Downloads

Download thousands of study notes,
question collections, GMAT Club’s
Grammar and Math books.
All are free!

Thank you for using the timer!
We noticed you are actually not timing your practice. Click the START button first next time you use the timer.
There are many benefits to timing your practice, including:

Re: Data sufficiency +- exponents question [#permalink]
10 Jun 2010, 13:22

9

This post received KUDOS

Expert's post

TheGmatTutor wrote:

Bunuel, I agree about S1. But on S2, that quantity y^2 will always be positive, correct?

No, not correct. \(y^2\) is not always positive, it's never negative: \(y^2\geq{0}\).

Inequality \(x^7*y^2*z^3>0\) to be true \(x\) and \(z\) must be either both positive or both negative (note that both positive or both negative excludes the possibility of either of them to be zero) AND \(y\) must not be zero. Because if \(y=0\), then \(x^7*y^2*z^3=0\). _________________

Answer is C. However, for reaching to the conclusion of YES/NO, we have to be certain that: (1) x, y, z are not ZERO, (2) [highlight]x and y are not NEGATIVE[/highlight]

Combined from (1) and (2) we can say that x, y, z are not ZERO. However, I think they are not helpful in deciding the certainty that x and y are not NEGATIVE.

Can some one please explain the answer?

First of all, a trick in the question is 'x, y, z are not ZERO' so good that you figured it. Next, we don't need to know that x and z are not negative. We need to know [highlight]whether they have the same sign or opposite signs[/highlight] because question asks you whether (x^7)(z^3) is positive. (Ignoring y for now) For the product to be positive, either both should be positive or both negative. Then, answer will be 'YES' For the product to be negative only one of them should be negative. Then answer will be 'NO' In either case, if we get a definite YES/NO, the statements will be sufficient. If xz> 0, then either x and z both are positive or both are negative. They have the same sign. So (x^7)(z^3) is positive. y, we know is not 0, so YES, (x^7)(y^2)(z^3) is greater than 0. Sufficient. _________________

Re: Data sufficiency +- exponents question [#permalink]
11 Jan 2011, 05:47

Bunuel wrote:

TheGmatTutor wrote:

Bunuel, I agree about S1. But on S2, that quantity y^2 will always be positive, correct?

No, not correct. \(y^2\) is not always positive, it's never negative: \(y^2\geq{0}\).

Inequality \(x^7*y^2*z^3>0\) to be true \(x\) and \(z\) must be either both positive or both negative (note that both positive or both negative excludes the possibility of either of them to be zero) AND \(y\) must not be zero. Because if \(y=0\), then \(x^7*y^2*z^3=0\).

Thanks a ton Bunuel , this example is the perfect for learning that while considering signs we should consider +ve , -ve and zero as well . Superb collection .

Response: x^7*y^2*z^3 is same as: (xz)*(x^6)*(y^2)*(z^2) xz is positive based on assumption #2. x^6 is (x^2)^3, since x^2 is positive for all real numbers, x^6 is also positive. z^2 is positive for all real numbers. y^2 is positive for all real numbers.

So product of 4 positive real numbers is also positive. SUFFICIENT.

Hence, B - Statement 2 Alone is Sufficient.

Per the gmatclub math25 test, the response is C - both statements together are sufficient. What incorrect assumptions am I making?

Thanks!

The red parts are not correct. Square of a number is nonnegative and not positive as you've written. So for (2) if y=0 then x^7*y^2*z^3=0.

Complete solution: Is x^7*y^2*z^3 > 0 ?

Inequality \(x^7*y^2*z^3>0\) to be true \(x\) and \(z\) must be either both positive or both negative (in order \(x^7*z^3\) to be positive) AND \(y\) must not be zero (in order \(x^7*y^2*z^3\) not to equal to zero).

(1) \(yz<0\) --> \(y\neq{0}\). Don't know about \(x\) and \(z\). Not sufficient.

(2) \(xz>0\) --> \(x\) and \(z\) are either both positive or both negative. Don't know about \(y\). Not sufficient.

Re: Is x^7*y^2*z^3 > 0 ? (1) yz < 0 (2) xz > 0 [#permalink]
23 Mar 2012, 05:03

3

This post received KUDOS

Expert's post

kraizada84 wrote:

x^7*y^2*z^3>0 y is positive, we need to know regarding x and z 1) no information about x insufficient

2) xz>0

either of them is negative hence

the answer to our question is no

sufficient

hence B

OA for this question is C, not B.

(2) \(xz>0\) means that\(x\) and \(z\) are either both positive or both negative. So, \(x^7*z^3 > 0\) but y can still be zero and in this case \(x^7*y^2*z^3=0\), hence this statement is not sufficient.

Re: Data sufficiency +- exponents question [#permalink]
13 Jun 2012, 11:24

Hi,

Simplifying the expression, \(x^7y^2z^3\), it can be written as, \((xz)^3x^4y^2\) or \((yz)^2x^6(xz)\)

Clearly, in both the expressions \(x^4y^2\) as well as \((yz)^2x^6\) are positive. Thus the sign of expression depends on sign of xz, but since value of y can be 0, Using (1), we can say y is not equal to 0.

Response: x^7*y^2*z^3 is same as: (xz)*(x^6)*(y^2)*(z^2) xz is positive based on assumption #2. x^6 is (x^2)^3, since x^2 is positive for all real numbers, x^6 is also positive. z^2 is positive for all real numbers. y^2 is positive for all real numbers.

So product of 4 positive real numbers is also positive. SUFFICIENT.

Hence, B - Statement 2 Alone is Sufficient.

Per the gmatclub math25 test, the response is C - both statements together are sufficient. What incorrect assumptions am I making?

Thanks!

The red parts are not correct. Square of a number is nonnegative and not positive as you've written. So for (2) if y=0 then x^7*y^2*z^3=0.

Complete solution: Is x^7*y^2*z^3 > 0 ?

Inequality \(x^7*y^2*z^3>0\) to be true \(x\) and \(z\) must be either both positive or both negative (in order \(x^7*z^3\) to be positive) AND \(y\) must not be zero (in order \(x^7*y^2*z^3\) not to equal to zero).

(1) \(yz<0\) --> \(y\neq{0}\). Don't know about \(x\) and \(z\). Not sufficient.

(2) \(xz>0\) --> \(x\) and \(z\) are either both positive or both negative. Don't know about \(y\). Not sufficient.

Response: x^7*y^2*z^3 is same as: (xz)*(x^6)*(y^2)*(z^2) xz is positive based on assumption #2. x^6 is (x^2)^3, since x^2 is positive for all real numbers, x^6 is also positive. z^2 is positive for all real numbers. y^2 is positive for all real numbers.

So product of 4 positive real numbers is also positive. SUFFICIENT.

Hence, B - Statement 2 Alone is Sufficient.

Per the gmatclub math25 test, the response is C - both statements together are sufficient. What incorrect assumptions am I making?

Thanks!

The red parts are not correct. Square of a number is nonnegative and not positive as you've written. So for (2) if y=0 then x^7*y^2*z^3=0.

Complete solution: Is x^7*y^2*z^3 > 0 ?

Inequality \(x^7*y^2*z^3>0\) to be true \(x\) and \(z\) must be either both positive or both negative (in order \(x^7*z^3\) to be positive) AND \(y\) must not be zero (in order \(x^7*y^2*z^3\) not to equal to zero).

(1) \(yz<0\) --> \(y\neq{0}\). Don't know about \(x\) and \(z\). Not sufficient.

(2) \(xz>0\) --> \(x\) and \(z\) are either both positive or both negative. Don't know about \(y\). Not sufficient.

Hi, When I solved this i get an answer as B. i.e. only 2nd statement is sufficient. But GMAT Club test says its C. Can anyone please help.

According to me, if xz > 0 then the inequality can be written as [(xz)^3 (x^4) (y^2)] Now since x^4 and y^2 are going to be positive always then the statement in itself gives us the answer.

Hi, When I solved this i get an answer as B. i.e. only 2nd statement is sufficient. But GMAT Club test says its C. Can anyone please help.

According to me, if xz > 0 then the inequality can be written as [(xz)^3 (x^4) (y^2)] Now since x^4 and y^2 are going to be positive always then the statement in itself gives us the answer.

Hi, When I solved this i get an answer as B. i.e. only 2nd statement is sufficient. But GMAT Club test says its C. Can anyone please help.

According to me, if xz > 0 then the inequality can be written as [(xz)^3 (x^4) (y^2)] Now since x^4 and y^2 are going to be positive always then the statement in itself gives us the answer.

Got it. Forgot to consider y = 0. As always. Thanks anyways.

Hello everyone! Researching, networking, and understanding the “feel” for a school are all part of the essential journey to a top MBA. Wouldn’t it be great... ...

Hey Everyone, I am launching a new venture focused on helping others get into the business school of their dreams. If you are planning to or have recently applied...