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Source: — Data Sufficiency |
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by djkvakin » Mon Dec 21, 2009 4:45 pm
please edit the post.
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by getso » Mon Dec 21, 2009 9:02 pm
Sorry...I have edited the post now..
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by papgust » Mon Dec 21, 2009 9:14 pm
IMO A

If square root (xy) is xy, then xy must be equal to 1 as root(1) = 1. But x and y could be any value. The question is x+y?

A. x = -1/2

As x*y = 1, y = 1/x = -2. x+y value can be found out. Sufficient.

B. y not equal to 0. But many possible values are there to satisy x*y=1. Insufficient.

Please share the OA.
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by Testluv » Mon Dec 21, 2009 10:09 pm
getso wrote:If square root of xy = xy, what is the value of x + y?

(1) x = -1/2
(2) y is not equal to zero
The question stem: square root xy = xy, therefore xy = x^2 * y^2

(1) x = -1/2

subbing in:

(-1/2) * y = (1/2)^2 * y^2

or

-y/2 = y^2/4

or

0 = y^2/4 + y/2

multiplying by 4 to eliminate fractions:

0 = 2y + y^2

factoring out y:

0 = y * (2 + y)

y = 0 or -2

Because we have multiple values for y, this statement is insufficient. The second statement is clearly insufficient because it leaves infinite values for y, and there is no info about x. Together, you know that y is -2 and that x is -1/2.

Apart, insufficient; together, sufficient.

Choose C.

@papgust: you forgot to consider that the equation in the question stem is also satisfied if either of x or y are zero. ;)
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by papgust » Mon Dec 21, 2009 10:40 pm
Thanks Testluv! My mistake..
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by getso » Tue Dec 22, 2009 10:12 am
Testluv wrote:
getso wrote:If square root of xy = xy, what is the value of x + y?

(1) x = -1/2
(2) y is not equal to zero
The question stem: square root xy = xy, therefore xy = x^2 * y^2

(1) x = -1/2

subbing in:

(-1/2) * y = (1/2)^2 * y^2

or

-y/2 = y^2/4

or

0 = y^2/4 + y/2

multiplying by 4 to eliminate fractions:

0 = 2y + y^2

factoring out y:

0 = y * (2 + y)

y = 0 or -2

Because we have multiple values for y, this statement is insufficient. The second statement is clearly insufficient because it leaves infinite values for y, and there is no info about x. Together, you know that y is -2 and that x is -1/2.

Apart, insufficient; together, sufficient.

Choose C.

@papgust: you forgot to consider that the equation in the question stem is also satisfied if either of x or y are zero. ;)
Thanks testluv.

xy = x^2 * y^2

I made a mistake of cancelling xy, on both sides of xy=x^2*y^2...
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by Testluv » Tue Dec 22, 2009 10:56 am
Hi Shobha,

cancelling xy on both sides ignores the fact that the equation is also satisfied if either of x or y are zero.
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