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Baffled by Some Algebra

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Source: — Data Sufficiency |
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by cunazza » Sat Feb 27, 2010 10:37 pm
arzanr wrote:Is x^2 equal to xy?

(1) x^2 - y^2 = (x+5)(y-5)
(2) x = y

Why is the answer B and not D?
From statement (1) we cannot answer the question. We know that x^2 - y^2 - xy - 5x + 5y - 25 and this is not enough to find out what xy is. INSUFF.
Statement (2) tells us that x = y, which means that x^2 is equal to xy because x and y ARE the SAME thing! SUFF.
So the correct answer is B.
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by arzanr » Sun Feb 28, 2010 8:27 am
But x^2-y^2 = (x+y)(x-y)

So if (x+y)(x-y) = (x+5)(y-5) doesn't this show that x=y and y=5?
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by Fiver » Mon Mar 01, 2010 6:48 am
arzanr wrote:But x^2-y^2 = (x+y)(x-y)

So if (x+y)(x-y) = (x+5)(y-5) doesn't this show that x=y and y=5?
Or
they could be x = -5 & y = 5.
In this case xy becomes negative.

And btw I glaced through this problem last evening and came up with this idea 24hrs since...
I'm improving; time for a drink i guess... :mrgreen:
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