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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] New post 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
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Re: DS Algebra [#permalink] New post 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] New post 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 :beat Laxie, your explanation makes spotting the trick seem so effortless. YOU RULE! :-D
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 [#permalink] New post 25 Mar 2006, 21:24
"C" it is!

using both we get x^3-4x = 0, which is given equation too!
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Re: DS Algebra [#permalink] New post 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
Z.jpg [ 12.93 KiB | Viewed 284 times ]

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Re: DS Algebra [#permalink] New post 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 :wink:

We only care about the value of the whole expression ( x^3- 4x) which has the value of 0
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Re: DS Algebra [#permalink] New post 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


Oops, your totally right :shock:
Ok, ok, I admit it is C.
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 [#permalink] New post 01 Apr 2006, 05:08
Its C.

I like the way u used graphs 2 solve... GMAT shud have some graph questions!
  [#permalink] 01 Apr 2006, 05:08
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What is the value of y + x^3 + x? (1) y = x (x - 3) (x + 3)

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