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# if xy>0, does (x-1)(y-1) = 1

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Intern
Joined: 26 Nov 2004
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if xy>0, does (x-1)(y-1) = 1 [#permalink]

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28 Nov 2004, 14:11
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Just wondering if the answer in the OG is correct for the following problem.

if xy>0, does (x-1)(y-1) = 1

1) x+y = xy
2) x = y

what do you guys think.
Director
Joined: 31 Aug 2004
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28 Nov 2004, 14:26
I pick A.

(1-x)(1-y)=1+xy-x-y=1+0=1

Numbers picking excludes statement 2
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28 Nov 2004, 14:49
twixt wrote:
I pick A.

(1-x)(1-y)=1+xy-x-y=1+0=1

Numbers picking excludes statement 2

Correct twixt, i my thinking was a little bit messed up, i was thinking something like a D for answer. It just struck my mind that it should be A.
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21 Dec 2004, 15:19
I pick b.
(x-1)(y-1) = 1
xy-x-y+1 = 1
xy-(x+y)= 1-1=0

Lets apply (1) means xy-xy = 0 but xy> 0.
lets apply (2) x= y means x^2-2x= 0.so x^2 = 2x x= 2=y.
xy>0 4>0.

please correct me if iam wrong
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21 Dec 2004, 21:38
No, you're wrong. It's definitely not B. Just because xy>0 doesn't mean that xy-(x+y) isn't 0. I'm pretty sure the answer is A.
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22 Dec 2004, 14:50
Stem (x-1)(y-1) = 1 can be expanded to
xy -x-y+1=1
or
xy = x+y

A) says exactly this

B) B says x=y, the stem will be valid only if (y-1)^2=1, but we don't know that. So this is not it

A for this.
22 Dec 2004, 14:50
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