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If xy = -6 , what is the value of xy(x+y ) ?

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If xy = -6 , what is the value of xy(x+y ) ? [#permalink] New post 07 Oct 2010, 05:46
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If xy = -6 , what is the value of xy(x+y ) ?

(1) x – y = 5
(2) x*y^2 = 18
[Reveal] Spoiler: OA
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Re: DS - Please explain [#permalink] New post 07 Oct 2010, 05:56
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agnok wrote:
If xy = -6 , what is the value of xy(x+y ) ?
(1) x – y = 5
(2) x*y square = 18


We're given xy = -6, so xy(x+y) = -6(x+y).

(1) x - y = 5 -> x = 5+y, so we have:

(5+y)(y) = -6
5y + y^2 = -6
y^2 + 5y + 6 = 0
y = -2 or y = -3.

Two solutions for y - insufficient.

(2) x(y^2) = 18. We know that y = \frac{-6}{x}, so we have x(\frac{-6}{x^2}) = 18. So:

x(\frac{36}{x^2}) = 18
\frac{36}{x} = 18
x = 2

Then y = -3, and xy(x+y) = -6(2-3) = 6. Sufficient.

B

Last edited by TehJay on 07 Oct 2010, 06:02, edited 1 time in total.
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Re: DS - Please explain [#permalink] New post 07 Oct 2010, 06:01
Expert's post
agnok wrote:
If xy = -6 , what is the value of xy(x+y ) ?
(1) x – y = 5
(2) x*y square = 18


If xy = -6 , what is the value of xy(x+y ) ?

Given: xy=-6.
Question: xy(x+y)=-6(x+y)=?

(1) x-y=5 --> x=y+5 --> xy=(y+5)y=-6 --> y^2+5y+6=0 --> either y=-3 and x=y+5=2 so xy(x+y)=-6(x+y)=6 OR y=-2 and x=y+5=3 so xy(x+y)=-6(x+y)=-6. Two different answers, hence not sufficient.

(2) x*y^2=18 --> xy*y=-6*y=18 --> y=-3 --> x=2 --> xy(x+y)=-6(x+y)=6. Sufficient.

Answer: B.
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Re: DS - Please explain [#permalink] New post 08 Oct 2010, 10:14
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ans is B :-)...well in GMAT maths ques are not tough bt tricky..mainly in DS
Re: DS - Please explain   [#permalink] 08 Oct 2010, 10:14
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