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Princeton Question

This topic has 2 member replies
f2001290 Master | Next Rank: 500 Posts
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Princeton Question

Post Sun May 06, 2007 1:05 am
Elapsed Time: 00:00
  • Lap #[LAPCOUNT] ([LAPTIME])
    For integers a and b, if sqrt(a^3 - a^2 - b) = 7, what is the value of a ?

    1. a^2 - a = 12
    2. b^2 - b = 2

    Please explain the solution for this problem.
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    jayhawk2001 Community Manager
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    Post Sun May 06, 2007 11:16 am
    f2001290 wrote:
    For integers a and b, if sqrt(a^3 - a^2 - b) = 7, what is the value of a ?

    1. a^2 - a = 12
    2. b^2 - b = 2

    Please explain the solution for this problem.
    Using (1), you get a^2 -a - 12 = 0 which implies a = 4 or a = -3.
    Hence insufficient.

    Using (2), b^2 -b -2 = 0 which implies b = 2 or b = -1

    a^3 - a^2 - b = 49
    a^2(a-1) = 49 + b

    We know a(a-1) should be even. so, b = -1

    So, a^2(a-1) = 48

    Only a=4 satisfies above eqn. sufficient.

    Hence B

    f2001290 Master | Next Rank: 500 Posts
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    Post Sun May 06, 2007 8:02 pm
    Jay Hawk

    Did you score a 800 in GMAT? Smile

    You don't leave any questions in PS unanswered

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