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Resistors connected in parallel

This topic has 1 expert reply and 3 member replies
LulaBrazilia Master | Next Rank: 500 Posts Default Avatar
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Resistors connected in parallel

Post Mon Mar 17, 2014 8:28 am
Elapsed Time: 00:00
  • Lap #[LAPCOUNT] ([LAPTIME])
    In an electric circuit, two resistors with resistances x and y are connected in parallel. In this case, if r is the combined resistance of these two resistors, then the reciprocal of r is equal to the sum of the reciprocals of x and y. What is r in terms of x and y?

    A) xy

    B) x+y

    C) 1/(x+y)

    D) xy/(x+y)

    E) (x+y)/xy

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    Abhishek009 Master | Next Rank: 500 Posts
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    Post Mon Mar 17, 2014 8:35 am
    LulaBrazilia wrote:
    In an electric circuit, two resistors with resistances x and y are connected in parallel. In this case, if r is the combined resistance of these two resistors, then the reciprocal of r is equal to the sum of the reciprocals of x and y. What is r in terms of x and y?
    Frame equations -

    LulaBrazilia wrote:
    the reciprocal of r is equal to the sum of the reciprocals of x and y. What is r in terms of x and y?
    1/r = 1/x + 1/y

    1/r = (x+y)/xy

    xy = r ( x + y )

    Or, r = xy / x + y

    Hence IMO answer is (D)

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    Abhishek

    GMAT/MBA Expert

    Post Mon Mar 17, 2014 8:39 am
    Do not confuse "r is the combined resistance of the two resistors" with "r is the sum of x and y". It is the next sentence that gives us the equation we need. "the reciprocal of r is the sum of the reciprocals of x and y" means that 1/r = 1/x + 1/y

    The answer is D. I go through the question in detail in the full solution below (taken from the GMATFix App).



    -Patrick

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    Post Tue Mar 18, 2014 12:02 am
    Hi LulaBrazilia,

    This question can be solved with TESTing Values.

    We're told that the reciprocal of R is equal to the SUM of the reciprocals of X and Y. This means….

    1/R = 1/X + 1/Y

    We're asked for the value of R in terms of X and Y

    If X = 2 and Y = 3, then we have…

    1/R = 1/2 + 1/3

    1/R = 3/6 + 2/6 = 5/6

    R = 6/5

    So we need an answer that = 6/5 when X = 2 and Y = 3.

    The only answer that matches is D

    GMAT assassins aren't born, they're made,
    Rich

    _________________
    Contact Rich at Rich.C@empowergmat.com

    Post Tue Mar 18, 2014 12:03 am
    Hi LulaBrazilia,

    This question can be solved with TESTing Values.

    We're told that the reciprocal of R is equal to the SUM of the reciprocals of X and Y. This means….

    1/R = 1/X + 1/Y

    We're asked for the value of R in terms of X and Y

    If X = 2 and Y = 3, then we have…

    1/R = 1/2 + 1/3

    1/R = 3/6 + 2/6 = 5/6

    R = 6/5

    So we need an answer that = 6/5 when X = 2 and Y = 3.

    The only answer that matches is D

    GMAT assassins aren't born, they're made,
    Rich

    _________________
    Contact Rich at Rich.C@empowergmat.com

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