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# What is the value of y + x^3 + x? (1) y = x (x - 3) (x + 3)

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What is the value of y + x^3 + x? (1) y = x (x - 3) (x + 3) [#permalink]  25 Mar 2006, 20:11
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What is the value of y + x^3 + x?

(1) y = x (x - 3) (x + 3)

(2) y = -5x
SVP
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Re: DS Algebra [#permalink]  25 Mar 2006, 20:20
GMATT73 wrote:
What is the value of S= y + x^3 + x?

(1) y = x (x - 3) (x + 3)

(2) y = -5x

(1) y= x( x^2-9) = x^3 - 9x
--> S= x^3- 9x + x^3 +x= 2*x^3 - 8x --> can't point out the concrete value of S -->insuff

(2) y= -5x --> S= x^3 - 4x --> insuff

(1) and (2):

we have y= x(x-3) ( x+3) = -5x --> x^3 - 9x = -5x --> x^3 - 4x= 0
from 2 we have: S= x^3 - 4x --> S= 0 --> suff

C it is.
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Re: DS Algebra [#permalink]  25 Mar 2006, 20:54
laxieqv wrote:
GMATT73 wrote:
What is the value of S= y + x^3 + x?

(1) y = x (x - 3) (x + 3)

(2) y = -5x

(1) y= x( x^2-9) = x^3 - 9x
--> S= x^3- 9x + x^3 +x= 2*x^3 - 8x --> can't point out the concrete value of S -->insuff

(2) y= -5x --> S= x^3 - 4x --> insuff

(1) and (2):

we have y= x(x-3) ( x+3) = -5x --> x^3 - 9x = -5x --> x^3 - 4x= 0
from 2 we have: S= x^3 - 4x --> S= 0 --> suff

C it is.

Duh Laxie, your explanation makes spotting the trick seem so effortless. YOU RULE!
SVP
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"C" it is!

using both we get x^3-4x = 0, which is given equation too!
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Re: DS Algebra [#permalink]  26 Mar 2006, 10:07
laxieqv wrote:
S= x^3 - 4x --> S= 0 --> suff

C it is.

Well that's not sufficient, since x^3 - 4x has 3 roots
at -2, 0, and +2. The graph looks like the Z of Zorro

So it is E.
Attachments

File comment: Graph of x^3 - 4x

Z.jpg [ 12.93 KiB | Viewed 398 times ]

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Re: DS Algebra [#permalink]  26 Mar 2006, 10:14
ccax wrote:
laxieqv wrote:
S= x^3 - 4x --> S= 0 --> suff

C it is.

Well that's not sufficient, since x^3 - 4x has 3 roots
at -2, 0, and +2. The graph looks like the Z of Zorro

So it is E.

But we don't need to consider the roots here

We only care about the value of the whole expression ( x^3- 4x) which has the value of 0
Manager
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Re: DS Algebra [#permalink]  26 Mar 2006, 10:20
laxieqv wrote:
We only care about the value of the whole expression ( x^3- 4x) which has the value of 0

Ok, ok, I admit it is C.
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Its C.

I like the way u used graphs 2 solve... GMAT shud have some graph questions!
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