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OG - DS - #97

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Mariela
 Post subject: OG - DS - #97
 Post Posted: Thu Jun 14, 2007 7:35 am 
What is the value of 5x^2 +4x-1?
1) x(x+2)=0
2)x=0

Statment 1 gives you 2 roots for x and the problem has an x^2, shouldn't that be the correct answer if you simplify the question as
x(5x+4)=1?

THanks,
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GMATPaduan
 Post subject: DS Response
 Post Posted: Thu Jun 14, 2007 11:40 am 
Rephrase the question to: What is X?
(1) x(x+2)= 0
Not sufficient: x could be 0, or x could be -2

2) x=0
Sufficient - gives a value for x
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Jeff
 Post subject: OG 11th ed Data sufficiency # 97
 Post Posted: Thu Jun 14, 2007 11:49 am 
Mariela -

The question statement asks what the value of 5x^2+4x-1 is...it does not tell you that 5x^2+4x-1=0, so you can't simplify it to x(5x+4)=1. To answer the question, you need to know the value of x, or the value of the factors (5x-1)(x+1), or know that one of those factors =0 (which would mean that 5x^2+4x-1=0).

Fact (2) gives us the value of x directly (x=0) so we then know that 5x^2+4x-1=-1 and (2) is sufficient by itself.

Fact (1) tells us that x=0 or x=-2. If x=0 then 5x^2+4x-1=-1. If x=-2 then 5x^2+4x-1= 11 so (1) is not sufficient by itself and the answer is B

Best,
Jeff
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dbernst
 Post subject: OG 11th ed Data sufficiency # 97
 Post Posted: Thu Jun 14, 2007 12:00 pm 
Offline
ManhattanGMAT Staff


Posts: 304
Jeff, thanks for the explanation. I was wondering where you have been!

Mariela, as I stated on your other submission, it is extremely important on Data Sufficiency to practice carefully rephrasing the original question. However, be sure to review your math fundamentals (especially with algebra) so that you don't accidentally go "too far" with your rephrase (i.e. break rules of mathematics). As Jeff noted, in this case you set the original equation equal to zero, but the problem never told you that this equation was equal to zero. On the GMAT, be sure to only utilize the information given. Don't assume anything!!!

-dan
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