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Gmatprep problem involving square-root

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jayhawk2001 Community Manager
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Gmatprep problem involving square-root

Post Sat Apr 14, 2007 2:29 pm
Elapsed Time: 00:00
  • Lap #[LAPCOUNT] ([LAPTIME])
    Another one from gmatprep.

    1 - doesn't tell us much

    2 - just tells us that x is negative

    sqrt ((x-3)^2) can yield either x-3 or 3-x. So, I chose E which clearly was
    wrong.

    I'll post the OA after seeing a few replies.
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    ajith Legendary Member
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    Post Sat Apr 14, 2007 7:29 pm
    sqrt refers to the positive root
    sqrt ((x-3)^2) can yield either x-3 or 3-x,whichever one is positive

    2 tells us that x is negative
    3-x is positive and thus the root.

    So 2 is clearly sufficient

    1 is not sufficient

    Smile
    B is the answer for me!

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    RAGS Senior | Next Rank: 100 Posts Default Avatar
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    Post Sun Apr 15, 2007 7:41 am
    The Q stem can be simplefied to say x-3 = 3-x ie 2x=6 or is x=3

    1 is sufficient

    2 says x is negative so cannot be equal to 3 hence sufficient

    so ans is D

    ajith Legendary Member
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    Post Sun Apr 15, 2007 8:10 am
    RAGS wrote:
    The Q stem can be simplefied to say x-3 = 3-x ie 2x=6 or is x=3
    wrong!

    sqrt(x-3)^2 is x-3 when x>3 and 3-x when x<3

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    jayhawk2001 Community Manager
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    Post Sun Apr 15, 2007 8:28 am
    ajith wrote:
    sqrt(x-3)^2 is x-3 when x>3 and 3-x when x<3
    Ok, this is the part I don't understand.

    Take x = -1, (-1-3)^2 = 16 and root-16 can be +4 or -4 right ?

    Why should the root operation yield only the positive root ?

    ajith Legendary Member
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    Post Sun Apr 15, 2007 9:53 am
    jayhawk2001 wrote:
    ajith wrote:
    sqrt(x-3)^2 is x-3 when x>3 and 3-x when x<3
    Ok, this is the part I don't understand.

    Take x = -1, (-1-3)^2 = 16 and root-16 can be +4 or -4 right ?

    Why should the root operation yield only the positive root ?
    A square root, also called a radical or surd, of x is a number r such that r^2==x. The function sqrt(x) is therefore the inverse function of f(x)==x^2 for x>=0.
    Source :http://mathworld.wolfram.com/SquareRoot.html


    Please refer the attached image
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    jayhawk2001 Community Manager
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    Post Sun Apr 15, 2007 11:31 am
    ajith wrote:
    A square root, also called a radical or surd, of x is a number r such that r^2==x. The function sqrt(x) is therefore the inverse function of f(x)==x^2 for x>=0.
    Source :http://mathworld.wolfram.com/SquareRoot.html
    Ah, ok. Official Guide Quantitative Review (Green book) page 24,
    section 7, also says the following --

    "Ever positive number n has two square roots, one positive and the
    other negative, but root-n denotes the positive number whose square
    is n".

    Darn, missed this.

    Just to summarize, x^2 = 9 implies x can be +3 or -3 but
    root-9 is always 3.

    leo Senior | Next Rank: 100 Posts Default Avatar
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    Post Mon Apr 16, 2007 6:33 pm
    I go for D. Whats the Answer?

    jayhawk2001 Community Manager
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    Post Mon Apr 16, 2007 9:14 pm
    OA is B

    RAGS Senior | Next Rank: 100 Posts Default Avatar
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    Post Tue Apr 17, 2007 9:16 am
    thanks for correcting me

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