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Post subject: GMAT Algebra (Data Sufficiency) Posted: Sat Jul 10, 2010 5:04 am 

Joined: Sun May 30, 2010 3:15 am Posts: 424

If a² = b, is 1 > a > 0? (1) 1 > b > 0 (2) a > b
A. Statement (1) BY ITSELF is sufficient to answer the question, but statement (2) by itself is not. B. Statement (2) BY ITSELF is sufficient to answer the question, but statement (1) by itself is not. C. Statements (1) and (2) TAKEN TOGETHER are sufficient to answer the question, even though NEITHER statement BY ITSELF is sufficient. D. Either statement BY ITSELF is sufficient to answer the question. E. Statements (1) and (2) TAKEN TOGETHER are NOT sufficient to answer the question, meaning that further information would be needed to answer the question.
(B) Statement (1) tells us that b is between 0 and 1, so we also know that a² is between 0 and 1 since b = a². If a is any number between 1 and 1, then a² will be between 0 and 1, so Statement (1) is insufficient.
Statement (2) tells us that a > b, and since a² = b, we know that a > a². This is only true for numbers between 0 and 1. The square of any number that is larger than 1 or negative will be larger than the number itself. So Statement (2) is sufficient.
Since Statement (1) is insufficient and Statement (2) is sufficient, the correct answer is choice (B). 
I think the answer should be letter D. Statement 1 should be sufficient since when I substitute 1/4 for b, it will yield 1/16 for a, so therefore 1>1/16>0.


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Gennadiy

Post subject: Re: GMAT Algebra (Data Sufficiency) Posted: Sat Jul 10, 2010 5:15 am 

Joined: Sun May 30, 2010 2:23 am Posts: 498

If you substitute 1/4 for b it gives you two options for a: 1/2 or 1/2.
So we can not definitely say is 1 > a > 0 or not.
Anyway, note, that by picking just one number you can prove that some statement is wrong, but you can not defenitely say that it is right if this number fits Ok. Here is a simple example for you:
Is a > 10 if a > 10? If we pick a = 20 it is true, but it doesn't prove that it is true for any other value of a that satisfies a > 10. E.g. it is not true for a = 20 though 20 > 10.


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questioner

Post subject: Re: GMAT Algebra (Data Sufficiency) Posted: Fri Feb 24, 2012 5:16 pm 

Joined: Sun May 30, 2010 3:15 am Posts: 424

I believe the answer to this one should be C.


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Gennadiy

Post subject: Re: GMAT Algebra (Data Sufficiency) Posted: Fri Feb 24, 2012 5:20 pm 

Joined: Sun May 30, 2010 2:23 am Posts: 498

questioner wrote: I believe the answer to this one should be C. C. Statements (1) and (2) TAKEN TOGETHER are sufficient to answer the question, even though NEITHER statement BY ITSELF is sufficient.In the explanation we've proven that statement (2) by itself IS sufficient. Therefore C can NOT be a correct answer.


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rquerub

Post subject: Re: GMAT Algebra (Data Sufficiency) Posted: Sun Jun 24, 2012 1:57 pm 

Joined: Fri Jun 22, 2012 8:53 am Posts: 2

"Statement (2) tells us that a > b and, since a² = b, then we know that a > a²".
Can you explain why you assume that a > a²?


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Gennadiy

Post subject: Re: GMAT Algebra (Data Sufficiency) Posted: Wed Jun 27, 2012 5:13 am 

Joined: Sun May 30, 2010 2:23 am Posts: 498

rquerub wrote: "Statement (2) tells us that a > b and, since a² = b, then we know that a > a²".
Can you explain why you assume that a > a²? Statement (2) is a > b . The question statement gives us b = a² . So we plug a² instead of b into a > b and get a > a² .


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questioner

Post subject: Re: GMAT Algebra (Data Sufficiency) Posted: Tue Jun 04, 2013 5:19 pm 

Joined: Sun May 30, 2010 3:15 am Posts: 424

(1) shows that a is a fraction for a² to be < 1


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Gennadiy

Post subject: Re: GMAT Algebra (Data Sufficiency) Posted: Tue Jun 04, 2013 5:29 pm 

Joined: Sun May 30, 2010 2:23 am Posts: 498

questioner wrote: (1) shows that a is a fraction for a² to be < 1 (1) shows that a is a number between 1 and 1 for a² to be less than 1. It can be 1/2 , for example.


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