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ds: Is X > Y?

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Director
Joined: 11 Jun 2007
Posts: 637
ds: Is X > Y? [#permalink]

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10 Dec 2007, 16:28
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Is X > Y?

(1) sqrt(X) > Y
(2) X^3 > Y

Do we consider sqrt(X) to be +ve?
Intern
Joined: 01 Dec 2007
Posts: 31
Re: ds: Is X > Y? [#permalink]

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10 Dec 2007, 16:46
eyunni wrote:
Is X > Y?

(1) sqrt(X) > Y
(2) X^3 > Y

Do we consider sqrt(X) to be +ve?

C

1) NS because x>y is true if x is 4 but not true if x is 1/4

2) NS because x>y is true if x is 2 but not true if x is -1/2

1&2 Sufficient - after plugging in 2 and 1/4.

1) says that x is postive so no need to test negative numbers.
Director
Joined: 11 Jun 2007
Posts: 637

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10 Dec 2007, 20:28
Manager
Joined: 01 Nov 2007
Posts: 82

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10 Dec 2007, 21:48
Yes - 1 makes X positive

solutions for sqrt a^2 can be +/- a but a^2 (in this case X) is always a positive number

Someone correct me if I'm wrong
Director
Joined: 09 Aug 2006
Posts: 755
Re: ds: Is X > Y? [#permalink]

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11 Dec 2007, 04:15
eyunni wrote:
Is X > Y?

(1) sqrt(X) > Y
(2) X^3 > Y

Do we consider sqrt(X) to be +ve?

Rule that comes out of this: If sqrt(x) > y and x^3 >y then x > y.
Director
Joined: 11 Jun 2007
Posts: 637

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11 Dec 2007, 08:20

(1) sqrt(X) > Y

Is sqrt(X) +ve in this question?
CEO
Joined: 17 Nov 2007
Posts: 3584
Concentration: Entrepreneurship, Other
Schools: Chicago (Booth) - Class of 2011
GMAT 1: 750 Q50 V40

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11 Dec 2007, 09:05
√X > Y is true only when √X is defined. Therefore x e (0,+∞]
Intern
Joined: 25 Nov 2007
Posts: 38

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11 Dec 2007, 09:06
eyunni wrote:

(1) sqrt(X) > Y

Is sqrt(X) +ve in this question?

sqrt(X) +ve if x is positive and -ve if x is negative..

C is the answer. Whats the OA??
Director
Joined: 11 Jun 2007
Posts: 637

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11 Dec 2007, 09:38
walker wrote:
shubhampandey wrote:
sqrt(X) +ve if x is positive and -ve if x is negative..

√X is always positive.

Very basic question: If √2 = +/- 1.414, why is √X always +ve?
CEO
Joined: 17 Nov 2007
Posts: 3584
Concentration: Entrepreneurship, Other
Schools: Chicago (Booth) - Class of 2011
GMAT 1: 750 Q50 V40

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11 Dec 2007, 13:03
eyunni wrote:
Very basic question: If √2 = +/- 1.414, why is √X always +ve?

√2 always equal +1.414 not -1.414.
maybe you think about questions like: x²=2, what is x?
in this case we have two solutions: x1=+√2 and x2=-√2, so x=+/-√2
but √2 itself is always positive.
SVP
Joined: 01 May 2006
Posts: 1796

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11 Dec 2007, 13:25
walker wrote:
eyunni wrote:
Very basic question: If √2 = +/- 1.414, why is √X always +ve?

√2 always equal +1.414 not -1.414.
maybe you think about questions like: x²=2, what is x?
in this case we have two solutions: x1=+√2 and x2=-√2, so x=+/-√2
but √2 itself is always positive.

I agree with Walker...

Also, there is the confusing definition of roots (that is related to x^2 = something).... Any number has 2 roots : sqrt(this number) and -sqrt(this number).
Director
Joined: 11 Jun 2007
Posts: 637

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11 Dec 2007, 13:57
Thanks Fig. OA is C. Also, drawing the graphs for (1) and (2) proves that x > y.
11 Dec 2007, 13:57
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