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Is xy < 1 ? (I) x + y = 1 (II) x^2 + y^2 = 1 [#permalink ]
12 Mar 2006, 09:28

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Is xy < 1 ?
(I) x + y = 1
(II) x^2 + y^2 = 1

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Laxie buddy, I'm scared of your problems

I'm getting D on this!

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vivek123 wrote:

Laxie buddy, I'm scared of your problems

I'm getting D on this!

hik, this is not my problem. I just picked it from somewhere

Btw, how is Tanvi, buddy?!!! ^.^ ....I'm looking forward to her new pix~

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I too am for D.
Statement 2 is definitely sufficient.
Statement 1 is also sufficient.

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laxieqv wrote:

Is xy < 1 ? (I) x + y = 1 (II) x^2 + y^2 = 1

the thing with D is that x or y can be negative (not both) then xy would be negative

like any value on the second and third quadrant of a cartesian plane

that's why I wouldn't pick D, I go with A on this one

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Can anyone give a numerical explanation?!

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(1) Can be:
x = 1/2, y = 1/2 xy < 1
x = 2, y = -1 xy < 1
No other case will result in xy < 1 and still satisfy x+y=1 (at least I can't think of any right now)
Sufficient
(2) Can be x = 1, y = 0 xy < 1. Can be x = 1/sqrt(2), y = 1/sqrt(2)xy > 1. Insufficient.
I go with A

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ywilfred wrote:

(1) Can be: x = 1/2, y = 1/2 xy < 1 x = 2, y = -1 xy < 1 No other case will result in xy < 1 and still satisfy x+y=1 (at least I can't think of any right now) Sufficient (2) Can be x = 1, y = 0 xy < 1. Can be x = 1/sqrt(2), y = 1/sqrt(2)xy > 1 . Insufficient. I go with A

Wilfred,

If x = 1/sqrt(2), y = 1/sqrt(2)

then xy = [1/sqrt(2)] * [1/sqrt(2)] = 0.5 < 1.

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oops

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laxieqv wrote:

vivek123 wrote:

Laxie buddy, I'm scared of your problems

I'm getting D on this!

hik, this is not my problem. I just picked it from somewhere

Btw, how is Tanvi, buddy?!!! ^.^ ....I'm looking forward to her new pix~

lexi what is the source. I also want to solve these problems...

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laxieqv wrote:

Is xy < 1 ? (I) x + y = 1 (II) x^2 + y^2 = 1

(1) x+y= 1 --> x= 1-y

---> xy= (1-y)*y= y- y^2 = - ( y^2 -y + 1/4) +1/4 = - ( y-1/2)^2 + 1/4

Because - (y-1/2) ^2 <= 0 ---> - (y-1/2) ^2 +1/4 <= 1/4 < 1

---> suff

(2) x^2+y^2 - 2xy = (x-y)^2 >= 0 ---> x^2+y^2 >= 2xy ---> 2xy<=1 -----> xy<=1/2< 1

---> suff

OA is C.

Good job, buddies~

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laxieqv wrote:

laxieqv wrote:

Is xy < 1 ? (I) x + y = 1 (II) x^2 + y^2 = 1

(1) x+y= 1 --> x= 1-y

---> xy= (1-y)*y= y- y^2 = - ( y^2 -y + 1/4) +1/4 = - ( y-1/2)^2 + 1/4

Because - (y-1/2) ^2 <= 0 ---> - (y-1/2) ^2 +1/4 <= 1/4 < 1

---> suff

(2) x^2+y^2 - 2xy = (x-y)^2 >= 0 ---> x^2+y^2 >= 2xy ---> 2xy<=1 -----> xy<=1/2< 1

---> suff

OA is C. Good job, buddies~

You mean "D"?

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vivek123 wrote:

laxieqv wrote:

laxieqv wrote:

Is xy < 1 ? (I) x + y = 1 (II) x^2 + y^2 = 1

(1) x+y= 1 --> x= 1-y

---> xy= (1-y)*y= y- y^2 = - ( y^2 -y + 1/4) +1/4 = - ( y-1/2)^2 + 1/4

Because - (y-1/2) ^2 <= 0 ---> - (y-1/2) ^2 +1/4 <= 1/4 < 1

---> suff

(2) x^2+y^2 - 2xy = (x-y)^2 >= 0 ---> x^2+y^2 >= 2xy ---> 2xy<=1 -----> xy<=1/2< 1

---> suff

OA is C. Good job, buddies~

You mean "D"?

Ah,hik, this is the consequence of staying up late till 4 am

OA is D.

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laxieqv wrote:

Is xy < 1 ? (I) x + y = 1 (II) x^2 + y^2 = 1

good question and good discussions.

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D it is. I freaked out for a moment when you posted the OA is C.

laxieqv wrote:

vivek123 wrote:

laxieqv wrote:

laxieqv wrote:

Is xy < 1 ? (I) x + y = 1 (II) x^2 + y^2 = 1

(1) x+y= 1 --> x= 1-y

---> xy= (1-y)*y= y- y^2 = - ( y^2 -y + 1/4) +1/4 = - ( y-1/2)^2 + 1/4

Because - (y-1/2) ^2 <= 0 ---> - (y-1/2) ^2 +1/4 <= 1/4 < 1

---> suff

(2) x^2+y^2 - 2xy = (x-y)^2 >= 0 ---> x^2+y^2 >= 2xy ---> 2xy<=1 -----> xy<=1/2< 1

---> suff

OA is C. Good job, buddies~

You mean "D"?

Ah,hik, this is the consequence of staying up late till 4 am

OA is D.

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Vivek and Laxie, did you give your GMAT already? Just curious on seeing the number of replies you both posted.

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chuckle wrote:

Vivek and Laxie, did you give your GMAT already? Just curious on seeing the number of replies you both posted.

Me? Not yet! Need to face the ghost sometime!

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oh..ok.. you are very accurate. So, I thought you already took it.