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If XY=-6, what is the value of XY(X+Y)? 1) X-Y=5 2) XY^2=18

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If XY=-6, what is the value of XY(X+Y)? 1) X-Y=5 2) XY^2=18 [#permalink] New post 21 Aug 2007, 21:50
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If XY=-6, what is the value of XY(X+Y)?

1) X-Y=5

2) XY^2=18

The book says the answer is B. I get D, as with #1, you can deduce Y to be -2 or -3 making X+Y=-5 regardless. Can anybody explain why/how 1) does not work?
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Re: OG Quanitative Review DS #96 [#permalink] New post 21 Aug 2007, 23:24
russiaoleg wrote:
If XY=-6, what is the value of XY(X+Y)?

1) X-Y=5

2) XY^2=18

The book says the answer is B. I get D, as with #1, you can deduce Y to be -2 or -3 making X+Y=-5 regardless. Can anybody explain why/how 1) does not work?


If you deduce Y to -2 and -3 then X must be 3 and 2 respectively.

So if X = 3 and Y = -2 then (X+Y) = 1.
If X = 2 and Y = -3 then (X+Y) = -1.

Hope this helps.
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Re: OG Quanitative Review DS #96 [#permalink] New post 21 Aug 2007, 23:26
russiaoleg wrote:
If XY=-6, what is the value of XY(X+Y)?

1) X-Y=5

2) XY^2=18

The book says the answer is B. I get D, as with #1, you can deduce Y to be -2 or -3 making X+Y=-5 regardless. Can anybody explain why/how 1) does not work?


Stat1: x-y=5

Since xy=-6, y=-6/x

Replacing the value of x in stat 1 and solving the quadratic equation x^2-5x+6=0 we get x=2 or x=3 which means that y=-3 or y=-2 (since xy=-6)

Using x=2 and y=-3 gives us a different value for xy(x+y) than using x=3 and y=-2; Insufficient

Stat2: xy^2=18

From the stem we get x=-6/y
From stat 2 we get x=18/y^2
Solving for y we get y=-3 meaning that x=2; suff

Answer: B
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 [#permalink] New post 23 Aug 2007, 15:44
Just think about it logically..

Facts give:

XY = -6

Option A

X- y = 5

Using the facts, you are going to be substituting X = -6/y and trying to solve a quadratic equation.. Which brings you 2 Values for Y ... so not a definitive answer//

But with Option B.. You get a defintive answer for Y..

Hope this helps.

Any time there is a quadratic equation , there are 2 values to keep in mind..

FYI..On first look, i assumed it was D too.. But it is B.
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Re: OG Quanitative Review DS #96 [#permalink] New post 23 Aug 2007, 16:05
russiaoleg wrote:
If XY=-6, what is the value of XY(X+Y)?

1) X-Y=5

2) XY^2=18

The book says the answer is B. I get D, as with #1, you can deduce Y to be -2 or -3 making X+Y=-5 regardless. Can anybody explain why/how 1) does not work?


A is not sufficient. then move to B, if B is not sufficient then no need to combine. But, if B is not suff as well then u gotta combine it.
It looks like it is D, but, since B is suff. to tell the value of XY(X+Y)
B is suff alone.
Re: OG Quanitative Review DS #96   [#permalink] 23 Aug 2007, 16:05
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If XY=-6, what is the value of XY(X+Y)? 1) X-Y=5 2) XY^2=18

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