It is currently 28 Jun 2017, 10:35

# Live Now:

### GMAT Club Daily Prep

#### 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

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

# Events & Promotions

###### Events & Promotions in June
Open Detailed Calendar

# M23 #14 - Incorrect OA ?

Author Message
Intern
Joined: 23 Oct 2010
Posts: 31
Location: London
WE 1: Consulting - 1.5 Yrs
WE 2: IB Finance - 5 Yrs
M23 #14 - Incorrect OA ? [#permalink]

### Show Tags

02 Jan 2011, 06:07
If line y=kx + b is parallel to line x=b + ky , which of the following must be true ?

A) k=b
B) k=1
C) b+k=0
D) |k|-1=0
E) k=-k
Intern
Joined: 23 Oct 2010
Posts: 31
Location: London
WE 1: Consulting - 1.5 Yrs
WE 2: IB Finance - 5 Yrs
Re: M23 #14 - Incorrect OA ? [#permalink]

### Show Tags

02 Jan 2011, 06:11
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)

Hence i think Ans should be B.
Manager
Joined: 20 Dec 2010
Posts: 168
Location: Stockholm, Sweden
Re: M23 #14 - Incorrect OA ? [#permalink]

### Show Tags

02 Jan 2011, 07:27

B is not true, |k| = 1 and k = 1 is not the same, rather |k| = 1 and {-k=1,k=1
_________________

12/2010 GMATPrep 1 620 (Q34/V41)
01/2011 GMATPrep 2 640 (Q42/V36)
01/2011 GMATPrep 3 700 (Q47/V39)
02/2011 GMATPrep 4 710 (Q48/V39)
02/2011 MGMAT CAT 1 650 (Q46/V32)
02/2011 MGMAT CAT 2 680 (Q46/V36)
02/2011 MGMAT CAT 3 710 (Q45/V41)

Intern
Joined: 23 Oct 2010
Posts: 31
Location: London
WE 1: Consulting - 1.5 Yrs
WE 2: IB Finance - 5 Yrs
Re: M23 #14 - Incorrect OA ? [#permalink]

### Show Tags

02 Jan 2011, 08:43
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.

Posted from my mobile device
Manager
Joined: 20 Dec 2010
Posts: 168
Location: Stockholm, Sweden
Re: M23 #14 - Incorrect OA ? [#permalink]

### Show Tags

02 Jan 2011, 09:26
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 ?
_________________

12/2010 GMATPrep 1 620 (Q34/V41)
01/2011 GMATPrep 2 640 (Q42/V36)
01/2011 GMATPrep 3 700 (Q47/V39)
02/2011 GMATPrep 4 710 (Q48/V39)
02/2011 MGMAT CAT 1 650 (Q46/V32)
02/2011 MGMAT CAT 2 680 (Q46/V36)
02/2011 MGMAT CAT 3 710 (Q45/V41)

Intern
Status: I will it!!!
Affiliations: Certified Credit Card Banking Professional
Joined: 20 Jul 2010
Posts: 35
Schools: Chicago Booth, ISB
WE 1: 6 years in IT BFSI Cards application
Re: M23 #14 - Incorrect OA ? [#permalink]

### Show Tags

02 Jan 2011, 09:46

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......
_________________

[align=]- Abhishek [/align]

Intern
Status: I will it!!!
Affiliations: Certified Credit Card Banking Professional
Joined: 20 Jul 2010
Posts: 35
Schools: Chicago Booth, ISB
WE 1: 6 years in IT BFSI Cards application
Re: M23 #14 - Incorrect OA ? [#permalink]

### Show Tags

02 Jan 2011, 09:48
Option B cannot be the answer as it has only +1 ....what about -1 ?
_________________

[align=]- Abhishek [/align]

Manager
Joined: 20 Dec 2010
Posts: 168
Location: Stockholm, Sweden
Re: M23 #14 - Incorrect OA ? [#permalink]

### Show Tags

02 Jan 2011, 09:49
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?
_________________

12/2010 GMATPrep 1 620 (Q34/V41)
01/2011 GMATPrep 2 640 (Q42/V36)
01/2011 GMATPrep 3 700 (Q47/V39)
02/2011 GMATPrep 4 710 (Q48/V39)
02/2011 MGMAT CAT 1 650 (Q46/V32)
02/2011 MGMAT CAT 2 680 (Q46/V36)
02/2011 MGMAT CAT 3 710 (Q45/V41)

Intern
Joined: 23 Oct 2010
Posts: 31
Location: London
WE 1: Consulting - 1.5 Yrs
WE 2: IB Finance - 5 Yrs
Re: M23 #14 - Incorrect OA ? [#permalink]

### Show Tags

02 Jan 2011, 10:12
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.
Manager
Joined: 10 Jan 2011
Posts: 237
Location: India
GMAT Date: 07-16-2012
GPA: 3.4
WE: Consulting (Consulting)
Re: M23 #14 - Incorrect OA ? [#permalink]

### Show Tags

30 Mar 2012, 02:16
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
_________________

-------Analyze why option A in SC wrong-------

Senior Manager
Status: May The Force Be With Me (D-DAY 15 May 2012)
Joined: 06 Jan 2012
Posts: 278
Location: India
Concentration: General Management, Entrepreneurship
Re: M23 #14 - Incorrect OA ? [#permalink]

### Show Tags

30 Mar 2012, 04:54
I also got B as the answer

But feel that D is right.

Anyone has the OA & Source ?
_________________

Giving +1 kudos is a better way of saying 'Thank You'.

Re: M23 #14 - Incorrect OA ?   [#permalink] 30 Mar 2012, 04:54
Similar topics Replies Last post
Similar
Topics:
M23 Q14 1 13 Mar 2010, 11:58
M23 #14 3 12 Aug 2009, 19:24
4 m23#17 21 01 Aug 2012, 09:16
6 M23 #35 22 13 Aug 2013, 01:20
26 m23 #33 19 21 Apr 2014, 05:45
Display posts from previous: Sort by

# M23 #14 - Incorrect OA ?

Moderator: Bunuel

 Powered by phpBB © phpBB Group and phpBB SEO Kindly note that the GMAT® test is a registered trademark of the Graduate Management Admission Council®, and this site has neither been reviewed nor endorsed by GMAC®.