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Thread: DS- Multiplication by zero

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    DS- Multiplication by zero

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    OG Quant Review - 2nd Edition - DS-Question 46


    What is the value of x^2 - y^2 ? (x square minus y square)


    (1) x - y = y + 2
    (2) x - y = 1 / (x+y)


    This is an official question. OA and answer explanation is available.
    My query is for second condition. How can we mutiply both sides by (x+y) unless it is specified that x+y is not equal to zero or x not equal to -y. Should we assume that denominator is not equal to zero in such GMAT questions ?

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    GRE-GMAT Instructor Brent Hanneson's Avatar
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    Quote Originally Posted by redpearl View Post
    OG Quant Review - 2nd Edition - DS-Question 46

    What is the value of x^2 - y^2 ? (x square minus y square)

    (1) x - y = y + 2
    (2) x - y = 1 / (x+y)

    This is an official question. OA and answer explanation is available.
    My query is for second condition. How can we mutiply both sides by (x+y) unless it is specified that x+y is not equal to zero or x not equal to -y. Should we assume that denominator is not equal to zero in such GMAT questions ?
    Statement 2 tells us that x-y = 1/(x+y). From this, we can conclude that x+y does not equal zero. If x+y did equal zero, then 1/(x+y) would be undefined in which case it couldn't equal some other value. So, knowing that 1/(x+y) equals some other value, it's safe to conclude that x-y does not equal zero.

    Cheers,
    Brent - GMAT Prep Now

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    IMO B. only statement 2 is needed.

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