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If xy = -6 , what is the value of xy(x+y ) ? (1) x y = 5 (2)

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If xy = -6 , what is the value of xy(x+y ) ? (1) x y = 5 (2) [#permalink] New post 18 Apr 2007, 10:00
If xy = -6 , what is the value of xy(x+y ) ?
(1) x – y = 5
(2) xy^2 = 18

My answer doesn't match with the answer given. Would like to know your answers.

Thanks
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 [#permalink] New post 18 Apr 2007, 10:52
answer is B

st2 is sufficient:
xy^2=18 = xy*y = (-6)*y hence y must be -3 from which you can go forward and get the value of xy(x+y)

st1: is insufficient
x-y=5 (together with xy=-6) has two solutions: x=2 y=-3 and x=3 y=-2 which lead to diferent answers - hence insufficient
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 [#permalink] New post 18 Apr 2007, 10:56
My answer is (B)

I can see that hobbit gave a full and detailed explanation so I will not explain again (same logic) - well done hobbit :-D
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 [#permalink] New post 18 Apr 2007, 13:55
I agree with hobbit B.
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 [#permalink] New post 18 Apr 2007, 19:01
B indeed! :)

xy^2=18
18=2*3^2

thus x=2 y=-3

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 [#permalink] New post 18 Apr 2007, 19:24
St1:
x = y+5
So
y(y+5) = -6
y^2 +5y + 6 = 0
y = -2 or y = -3

If y = -2, x = 3 --> xy(x+y) = -6
If y = -3, x = 2 --> xy(x+y) = -30

Insufficient.

St2:
xy^2 = 18
x = 18/y^2

So
(18/y^2)(y) = 6
18/y = 6
y = 3.

So x = 2. One (x,y) pair will give only one result for xy(x+y).

sufficient.

Ans B
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Re: Conflicting answers [#permalink] New post 18 Apr 2007, 19:55
jainvik7 wrote:
If xy = -6 , what is the value of xy(x+y ) ?
(1) x – y = 5
(2) xy^2 = 18

My answer doesn't match with the answer given. Would like to know your answers.

Thanks


B.

x-y=5 gives 2 sets of solutions
while xy^2=18 gives only 1
Re: Conflicting answers   [#permalink] 18 Apr 2007, 19:55
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