Last visit was: 25 Apr 2026, 22:05 It is currently 25 Apr 2026, 22:05
Home
Messages
Tests
Schools
Events
Close
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
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.
Go to My Error Log Learn more
Close
Request Expert Reply
Confirm Cancel
User avatar
pzazz12
User avatar
Joined: 22 Sep 2010
Last visit: 25 Oct 2010
Posts: 59
Own Kudos:
1,251
Posts: 59
Kudos: 1,251
If f(x)=(x-1)/(x+1), then f(2x) is equal to 01 Oct 2010, 04:37
Kudos
Add Kudos
Bookmarks
Bookmark this Post
00:00
Start Timer
Pause Timer
Resume Timer
Show Answer
Hide
Show
History
A
Be sure to select an answer first to save it in the Error Log before revealing the correct answer (OA)!

Difficulty:

(N/A)

Question Stats:

50% (01:58) correct 50% (02:43) wrong based on 4 sessions

History

Date
Time
Result
Not Attempted Yet
If f(x)=(x-1)/(x+1), then f(2x) is equal to

A. (3f(x)+1)/(f(x)+3)
B. (f(x)+1)/(f(x)+3)
C. (f(x)+3)/(f(x)+1)
D. (f(x)+3)/(f(x)+2)
E. None of these.



Archived Topic
Hi there,
This topic has been closed and archived due to inactivity or violation of community quality standards. No more replies are possible here.
Where to now? Join ongoing discussions on thousands of quality questions in our Problem Solving (PS) Forum
Still interested in this question? Check out the "Best Topics" block below for a better discussion on this exact question, as well as several more related questions.
Thank you for understanding, and happy exploring!
ShowHide Answer
Official Answer
A
User avatar
Bunuel
User avatar
Math Expert
Joined: 02 Sep 2009
Last visit: 25 Apr 2026
Posts: 109,830
Own Kudos:
811,298
Given Kudos: 105,886
Products:
GMAT Club Tests Forum Quiz
Expert
Expert reply
Active GMAT Club Expert! Tag them with @ followed by their username for a faster response.
Posts: 109,830
Kudos: 811,298
If f(x)=(x-1)/(x+1), then f(2x) is equal to 01 Oct 2010, 06:23
Kudos
Add Kudos
Bookmarks
Bookmark this Post
pzazz12
If f(x)=(x-1)/(x+1), then f(2x) is equal to

A. (3f(x)+1)/(f(x)+3)
B. (f(x)+1)/(f(x)+3)
C. (f(x)+3)/(f(x)+1)
D. (f(x)+3)/(f(x)+2)
E. None of these.
Show more

Number plugging is the best way for this problem.

For \(x=0\) --> \(f(x)=\frac{x-1}{x+1}=-1\) and \(f(2x)=\frac{2x-1}{2x+1}=-1\). So we should check which of the following choices equals to \(-1\), taking into account that \(f(x)=-1\).

A. \(\frac{3f(x)+1}{f(x)+3}=-1\) --> correct;

B. \(\frac{f(x)+1}{f(x)+3}=0\neq{-1}\);

C. \(\frac{f(x)+3}{f(x)+1}=undefined\neq{-1}\);

D. \(\frac{f(x)+3}{f(x)+2}=2\neq{-1}\);

E. None of these.

It might happen that for some choices of \(x\) other options may be "correct" as well (for example if \(x=1\) then A and B are both correct). If this happens just pick some other number for \(x\) and check again these "correct" options only.

Questions about the same concept with the different solutions:
function-85751.html?hilit=following%20functions%20positive#p644387
functions-problem-need-help-93184.html?hilit=function#p782040

Hope it helps.
User avatar
shrouded1
User avatar
Retired Moderator
Joined: 02 Sep 2010
Last visit: 29 Apr 2018
Posts: 608
Own Kudos:
3,232
Given Kudos: 25
Location: London
Products:
My Reviews
Posts: 608
Kudos: 3,232
If f(x)=(x-1)/(x+1), then f(2x) is equal to 01 Oct 2010, 07:07
Kudos
Add Kudos
Bookmarks
Bookmark this Post
Bunuel
pzazz12
If f(x)=(x-1)/(x+1), then f(2x) is equal to

A. (3f(x)+1)/(f(x)+3)
B. (f(x)+1)/(f(x)+3)
C. (f(x)+3)/(f(x)+1)
D. (f(x)+3)/(f(x)+2)
E. None of these.

Number plugging is the best way for this problem.

For \(x=0\) --> \(f(x)=\frac{x-1}{x+1}=-1\) and \(f(2x)=\frac{2x-1}{2x+1}=-1\). So we should check which of the following choices equals to \(-1\), taking into account that \(f(x)=-1\).

A. \(\frac{3f(x)+1}{f(x)+3}=-1\) --> correct;

B. \(\frac{f(x)+1}{f(x)+3}=0\neq{-1}\);

C. \(\frac{f(x)+3}{f(x)+1}=undefined\neq{-1}\);

D. \(\frac{f(x)+3}{f(x)+2}=2\neq{-1}\);

E. None of these.

It might happen that for some choices of \(x\) other options may be "correct" as well (for example if \(x=1\) then A and B are both correct). If this happens just pick some other number for \(x\) and check again these "correct" options only.

Questions about the same concept with the different solutions:
function-85751.html?hilit=following%20functions%20positive#p644387
functions-problem-need-help-93184.html?hilit=function#p782040

Hope it helps.
Show more

There is a flaw in this argument. The method of plugging in values is good to eliminate choices but it is not good to conclude on the answer with 100% certainty. So if you are given a question with 5 choices and you can eliminate 4 of them, you have got the answer. But in this case, one of the answers is "none of the above". Which means by eliminating b,c and d. All you have shown is that (a) can be a solution or there is no solution since all we know is that f(2x) as given by expression (a) is correct only for the special case x=0.

So you need to be more careful when there is a "none of the above" in the argument.

A quick check will suffice to guarantee the answer :

\(\frac{3f(x)+1}{f(x)+3} = \frac{3x-3+x+1}{x-1+3x+3} = \frac{4x-2}{4x+2} = \frac{2x-1}{2x+1} = f(2x)\)
User avatar
Bunuel
User avatar
Math Expert
Joined: 02 Sep 2009
Last visit: 25 Apr 2026
Posts: 109,830
Own Kudos:
811,298
Given Kudos: 105,886
Products:
GMAT Club Tests Forum Quiz
Expert
Expert reply
Active GMAT Club Expert! Tag them with @ followed by their username for a faster response.
Posts: 109,830
Kudos: 811,298
If f(x)=(x-1)/(x+1), then f(2x) is equal to 01 Oct 2010, 07:28
Kudos
Add Kudos
Bookmarks
Bookmark this Post
shrouded1
Bunuel
pzazz12
If f(x)=(x-1)/(x+1), then f(2x) is equal to

A. (3f(x)+1)/(f(x)+3)
B. (f(x)+1)/(f(x)+3)
C. (f(x)+3)/(f(x)+1)
D. (f(x)+3)/(f(x)+2)
E. None of these.

Number plugging is the best way for this problem.

For \(x=0\) --> \(f(x)=\frac{x-1}{x+1}=-1\) and \(f(2x)=\frac{2x-1}{2x+1}=-1\). So we should check which of the following choices equals to \(-1\), taking into account that \(f(x)=-1\).

A. \(\frac{3f(x)+1}{f(x)+3}=-1\) --> correct;

B. \(\frac{f(x)+1}{f(x)+3}=0\neq{-1}\);

C. \(\frac{f(x)+3}{f(x)+1}=undefined\neq{-1}\);

D. \(\frac{f(x)+3}{f(x)+2}=2\neq{-1}\);

E. None of these.

It might happen that for some choices of \(x\) other options may be "correct" as well (for example if \(x=1\) then A and B are both correct). If this happens just pick some other number for \(x\) and check again these "correct" options only.

Questions about the same concept with the different solutions:
function-85751.html?hilit=following%20functions%20positive#p644387
functions-problem-need-help-93184.html?hilit=function#p782040

Hope it helps.

There is a flaw in this argument. The method of plugging in values is good to eliminate choices but it is not good to conclude on the answer with 100% certainty. So if you are given a question with 5 choices and you can eliminate 4 of them, you have got the answer. But in this case, one of the answers is "none of the above". Which means by eliminating b,c and d. All you have shown is that (a) can be a solution or there is no solution since all we know is that f(2x) as given by expression (a) is correct only for the special case x=0.

So you need to be more careful when there is a "none of the above" in the argument.

A quick check will suffice to guarantee the answer :

\(\frac{3f(x)+1}{f(x)+3} = \frac{3x-3+x+1}{x-1+3x+3} = \frac{4x-2}{4x+2} = \frac{2x-1}{2x+1} = f(2x)\)
Show more

Yes, this is a common problem with number plugging. Though I've never seen a GMAT question with the trap you mentioned. So if one is short at time and not good at algebraic solutions I'd still recommend number plugging for such problems rather then: \(\frac{3f(x)+1}{f(x)+3} =\frac{\frac{3x-3}{x+1}+1}{\frac{x-1}{x+1}+3}= \frac{\frac{3x-3+x+1}{x+1}}{\frac{x-1+3x+3}{x+1}}=\frac{3x-3+x+1}{x-1+3x+3} =\frac{4x-2}{4x+2} = \frac{2x-1}{2x+1} = f(2x)\).

Also remember that we will be lucky if we start with A as we'll get the correct answer right away but what if we start with other choices? It'll take too much time.



Archived Topic
Hi there,
This topic has been closed and archived due to inactivity or violation of community quality standards. No more replies are possible here.
Where to now? Join ongoing discussions on thousands of quality questions in our Problem Solving (PS) Forum
Still interested in this question? Check out the "Best Topics" block above for a better discussion on this exact question, as well as several more related questions.
Thank you for understanding, and happy exploring!
Moderator:
Bunuel
Math Expert
109830 posts