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f(x) = f(1-x) function problem q

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czechchamp Rising GMAT Star Default Avatar
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f(x) = f(1-x) function problem q Post Sun Jul 06, 2008 2:13 pm
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  • Lap #[LAPCOUNT] ([LAPTIME])
    f(x) = f(1-x)

    A f(x) = 1-x
    B f(x) = 1-x^2
    C f(x) = x^2 - (1-x)^2
    D d. f(x) = x^2(1-x)^2 EDITED
    E f(x) = x / 1-x

    The correct answer is D. However, can someone elaborate on how to approach the problem? If I substitute 2 in D I get f(2) = 4 (1-2)^2 =>
    2 = 4. What am I missing?



    Last edited by czechchamp on Mon Jul 07, 2008 6:40 am; edited 1 time in total

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    sibbineni Really wants to Beat The GMAT! Default Avatar
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    Post Sun Jul 06, 2008 7:10 pm
    f(x) = f(1-x)

    let us consider the choices:

    option D
    f(x)=x^2*(1-x)^2----(1)

    substitute 1-x in the place of x in equation 1we have
    f(1-x)=(1-x)^2*(1-(1-x))^2
    ==>(1-x)^2*(x)^2==>x^2*(1-x)^2----(2)

    1=2

    so Option D only satisfies


    (OR)


    The correct answer is D. However, can someone elaborate on how to approach the problem? If I substitute 2 in D I get f(2) = 4 (1-2)^2 =>
    2 = 4. What am I missing?



    f(x)=x^2*(1-x)^2 ----(a)

    substitute 2 in equation (a)
    ==>4(1-2)^2==>4

    f(1-x)==>f(1-2)==>f(-1)

    so substitute f(-1 ) in equation (a)

    ==(-1)^2(1-(-1))^2==>2^2==>4


    so D is the answer

    Thanked by: czechchamp, phxccie
    moontashir Just gettin' started! Default Avatar
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    Post Sun Jul 06, 2008 10:03 pm
    is it a typo...option D doesnt seem to be right
    it says (1-x)

    Whereas in the solution it sez (1-x)^2

    Please correct me if im wrong
    im just a newbie...

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    sibbineni Really wants to Beat The GMAT! Default Avatar
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    Post Mon Jul 07, 2008 4:57 am
    czechchamp wrote

    If I substitute 2 in D I get f(2) = 4 (1-2)^2 =>
    2 = 4. What am I missing?

    so,option D should be

    f(x) = x^2 (1-x)^2

    i think he forgot to write ^2 in option D

    czechchamp Rising GMAT Star Default Avatar
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    Post Mon Jul 07, 2008 6:39 am
    sibbineni wrote:
    czechchamp wrote

    If I substitute 2 in D I get f(2) = 4 (1-2)^2 =>
    2 = 4. What am I missing?

    so,option D should be

    f(x) = x^2 (1-x)^2

    i think he forgot to write ^2 in option D
    Yes, indeed, I keyed it in wrong. The option D as noted above should be f(x) = x^2(1-x)^2

    Thanks for your explanation! Smile

    jsl Really wants to Beat The GMAT! Default Avatar
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    Post Mon Jul 07, 2008 7:21 am
    on this type of question - is the best/quickest way to solve this equation to plug in the functions into each of the answer choices? Is there a special GMAT shortcut or is this question painfully slow?

    aj5105 GMAT Titan Default Avatar
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    Post Wed Oct 08, 2008 2:56 am
    As I have learnt,depends form problem to problem.

    Here just substitute (1-x) for all functions.
    We get f(x)=f(1-x)=f(x) right away.

    510_At_It_Again Just gettin' started! Default Avatar
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    Post Sat May 16, 2009 3:41 am
    is there a more simple way to word this question? What is being asked is very convoluted.

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    silentdud Just gettin' started! Default Avatar
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    Post Fri May 29, 2009 12:32 pm
    [quote="aj5105"]As I have learnt,depends form problem to problem.

    Here just substitute (1-x) for all functions.
    We get f(x)=f(1-x)=f(x) right away.


    EDIT: actually that seems to be the fastest way

    vineetbatra GMAT Destroyer!
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    Post Sat Jun 27, 2009 1:29 pm
    I am having a lot of trouble understanding these type of a problems. Can someone please explain this in the most rudimentary way, from the basics of what is f(x) to what the question is seeking.

    It will be greatly appreciated.

    Thanks,

    Vineet

    Matmasi Just gettin' started! Default Avatar
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    Post Fri Aug 28, 2009 10:41 am
    Hi, I had hard times also.
    The question gives you 5 different functions and asks for which of the functions plugging in the value of x gives the same result of plugging (1-x).

    basically, just substitute (1-x)at x and see if they give you the same result:
    for example if you look at the first function:
    F(x)=1-x
    F(1-x) = 1-(1-x)=-x
    They give you different results so this functions is not that function that gives you the same result using x or (1-x).

    Let's have a look to the right one (D).
    F(X)= (x^2) * (1-x)^2
    F(1-X)= (1-x)^2 * ((1-(1-x))^2 = (1-x)^2 * (-x)^2 = (1-x)^2 * x^2
    So you get that same equation using x or (1-x).

    Goof luck for your studies.
    Mat

    CarpeDiem2011 Just gettin' started! Default Avatar
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    Post Mon Nov 16, 2009 1:25 pm
    I'm not sure I understand this.

    So we plug in x-1 for x for all the equations. Then what are we looking out for that gives D as the answer?

    Thanks!

    papgust Community Manager
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    Post Mon Mar 22, 2010 6:07 pm
    I'm sure that this question has been discussed many a times. But everywhere, plugging in seems the only option to solve.

    How do you solve this without plugging in?

    Mashoka Just gettin' started! Default Avatar
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    Post Mon May 24, 2010 4:37 pm
    The question is asking for (at which of the following f(x)=f(1-x)).
    and that could only be true by using the formula of choice D
    but what is worth to be asked: How do I solve this problem in less than 2 minutes!!!
    I think if you face this kind of question, try to make a guess, this kind of question is to make the probability of one scoring
    800 is very low!

    Thouraya Really wants to Beat The GMAT! Default Avatar
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    Post Sat Jun 19, 2010 4:59 am
    We would appreciate it if an EXPERT may inspire us on how to tackle those types of problems (functions) on the gmat,and how to solve this one specifically.

    THANKS!

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