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: M23 #14 - Incorrect OA ? [#permalink]
02 Jan 2011, 05:11

The OA is D.

OE : For lines to be parallel, their slopes must be equal. The second equation can be rewritten as y = x/k - b/k . Because slopes must be equal, k=1/k or k^2=1 or |k| = 1.

But for k=-1 the 2 equations are effectively the same (i.e. Not parallel)

Re: M23 #14 - Incorrect OA ? [#permalink]
02 Jan 2011, 07:43

Yes, when you solve the problem there will be 2 possible solutions to k, -1 and 1. But look at what is being asked in the q, are these lines parallel. And for k=-1, these lines are essentially the same y = -x + b. Hence not parallel.

Re: M23 #14 - Incorrect OA ? [#permalink]
02 Jan 2011, 08:26

TheBirla wrote:

Yes, when you solve the problem there will be 2 possible solutions to k, -1 and 1. But look at what is being asked in the q, are these lines parallel. And for k=-1, these lines are essentially the same y = -x + b. Hence not parallel.

Posted from my mobile device

Yeah you are right, sorry. Maybe someone can shed some light on this ? _________________

Re: M23 #14 - Incorrect OA ? [#permalink]
02 Jan 2011, 08:46

D is the right answer

y=kx+b y=x/k - b/k

Since slopes are equal k = 1/k k^2=1 k=+1 or -1 Therefore |k| = 1 ---> |k|-1 = 0

If k=1 , we knw hw the equs would look like

y=x+b y=x-b

If k=-1 , then two equations would look like

y = -x + b y = -x +b

In both the scenarios the co-efficient of x is same , so the lines are parallel , the constant b can be +ve or -ve ....that does not matter . if a line is parallel , it does not mean that the constants must have the same sign . It depends on in which quadrant the lines are . A line passing through 1st and 4th quadrant can be parallel to line passing through 2nd and 3rd quadrant . In such scenarios , the slope of both the lines would be equal BUT NOT the constants...... _________________

Re: M23 #14 - Incorrect OA ? [#permalink]
02 Jan 2011, 08:49

ysr wrote:

Option B cannot be the answer as it has only +1 ....what about -1 ?

The question is whether a line can be both parallell and equal at the same time. It's obvious that if the lines are equal they have the same slope.

The common definition would be that two lines are parallell if they have the same slope and do not intersect. Two equal lines would intersect at every point, so could you really say that two identical lines are parallell? _________________

Re: M23 #14 - Incorrect OA ? [#permalink]
02 Jan 2011, 09:12

ysr wrote:

If k=-1 , then two equations would look like

y = -x + b y = -x +b

In both the scenarios the co-efficient of x is same , so the lines are parallel , the constant b can be +ve or -ve ....that does not matter . if a line is parallel , it does not mean that the constants must have the same sign . It depends on in which quadrant the lines are . A line passing through 1st and 4th quadrant can be parallel to line passing through 2nd and 3rd quadrant . In such scenarios , the slope of both the lines would be equal BUT NOT the constants......

I agree that the sign of constant does not matter, but the constant itself does. And as Mackieman has pointed out already, if the constant of 2 parallel lines is the same, they are not parallel by definition, i.e. they are identical lines, BUT not parallel.

Re: M23 #14 - Incorrect OA ? [#permalink]
30 Mar 2012, 01:16

TheBirla wrote:

The OA is D.

OE : For lines to be parallel, their slopes must be equal. The second equation can be rewritten as y = x/k - b/k . Because slopes must be equal, k=1/k or k^2=1 or |k| = 1.

But for k=-1 the 2 equations are effectively the same (i.e. Not parallel)

Hence i think Ans should be B.

I also have the same question. Can anyone put more light on it _________________