800score.com http://www.800score.com/forum/ 

GMAT Algebra (Data Sufficiency) http://www.800score.com/forum/viewtopic.php?f=3&t=3677 
Page 1 of 1 
Author:  questioner [ Tue May 01, 2012 2:24 am ] 
Post subject:  GMAT Algebra (Data Sufficiency) 
What is the value of ab? (1) a + b = a (2) a + b = 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. (C) Statement (1) is a + b = a. If 0 > a then a = a. So the equation transforms into a + b = a. This yields b = 2a. Therefore ab = 2a². But we do NOT know the value of a. So statement (1) by itself is NOT sufficient. Statement (2) is a + b = b. Similarly to the previous reasoning, if 0 > b then b = b. So the equation transforms into a – b = b. This yields a = 2b. Therefore ab = 2b². But we do NOT know the value of b. So statement (2) by itself is NOT sufficient. If we use the both statements together, we can add the equations. a + b + a + b = a + b a + b = 0 Any absolute value is nonnegative. So the sum equals 0 if and only if each absolute value is 0. Therefore a = 0, b = 0 and ab = 0. Statements (1) and (2) taken together are sufficient to answer the question, even though neither statement by itself is sufficient. The correct answer is C.  I did not understand the concept of a = a. 
Author:  Gennadiy [ Tue May 01, 2012 2:37 am ] 
Post subject:  Re: t.1, qt.24: absolute values. 
Quote: I did not understand the concept of a = a. Take a look at the numerical example:4 = 4 4 = (4) = 4 Now in general, if a is a positive number, then a = a if a is a negative number, then a = a, because (a) is positive. 
Author:  questioner [ Thu May 17, 2012 10:30 am ] 
Post subject:  Re: t.1, qt.24: absolute values. 
This answer is really weird. In the question, it is not mentioned that "a" or "b" is a constant – which means " a + b = 0 " does not guarantee that a and b are "0". It could mean a = b. 
Author:  Gennadiy [ Thu May 17, 2012 10:51 am ] 
Post subject:  Re: t.1, qt.24: absolute values. 
Quote: In the question, it is not mentioned that "a" or "b" is a constant … a and b are variables.Quote: … " a + b = 0 " does not guarantee that a and b are "0" … Any absolute value is nonnegative. If a and b were not zeroes, then a would be positive and b would be positive.positive + positive > 0 That would contradict the equation. Even if only one variable was not zero, the left side would still be positive. So the both variables can possess only one value, 0. Quote: … It could mean a = b. If to plug in a = b we will get b + b as the left side of the equation. It is not necessarily 0. Plug any nonzero value for b, for example b = 2 yields 2 + 2 = 2 + 2 = 4. So the conclusion "It could mean a = b" is not correct.If to solve the equation b + b = 0: The absolute values b and b are the same (equal), so the equation transforms into 2b = 0 b = 0 
Author:  questioner [ Fri May 25, 2012 3:08 pm ] 
Post subject:  Re: t.1, qt.24: absolute values. 
Why is answer D incorrect? Statement (1) tells us that b is 0 and a must be positive, therefore ab = 0 (knowing b is 0 would be sufficient to answer the question and b has to equal 0 for the statement to be true). Statement (2) tells us that a is 0 and b must be positive, therefore ab = 0. (knowing a is 0 would be sufficient to answer the question and a has to equal 0 for the statement to be true). Thank you. 
Author:  Gennadiy [ Fri May 25, 2012 3:14 pm ] 
Post subject:  Re: t.1, qt.24: absolute values. 
Quote: Statement (1) tells us that b is 0 and a must be positive No, it does NOT. For example, take a = 3 and b = 6:3 – 6 = 3 The same is with the reasoning for Statement (2). 
Page 1 of 1  All times are UTC  5 hours [ DST ] 
Powered by phpBB © 2000, 2002, 2005, 2007 phpBB Group http://www.phpbb.com/ 