
It is currently Wed Jun 26, 2019 11:03 am

View unanswered posts  View active topics

Page 1 of 1

[ 10 posts ] 

Author 
Message 
questioner

Post subject: GMAT Algebra (Data Sufficiency) Posted: Mon Jan 24, 2011 4:55 pm 

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

If ax² + bx³ = 5, where a and b are nonzero numbers, what is the value of a + b? (1) ax² = a (2) bx³ = 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) The important thing to remember here is that, when a number is squared, it will always result in a positive, regardless of whether the original number was negative or positive. This is not the case with a number that is cubed. A cube will only be positive if the original integer was positive; otherwise, it will be negative. That having been said, statement (1) is insufficient. While we know that x can be nothing other than 1 or 1 in order for ax² to equal a, we are not told specifically if x is 1 or 1.
Statement (2), however, is sufficient. Because x is cubed, we know that it had to be 1 from the beginning. Statement (2) tells us that a + b = 5. The answer is (B).
For more information on this type of questions, order the online prep course by clicking the following link:

The right answer is D. ax² = a, so x² = 1, so x = 1. And so it is similar to statement (2).


Top 


Gennadiy

Post subject: Re: math (test 1, question 28): algebra. Posted: Mon Jan 24, 2011 5:12 pm 

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

ax² = a x² = 1 x = 1 or x = 1
If x = 1, then ax² + bx³ = a × 1² + b × 1³ = a + b = 5
If x = 1, then ax² + bx³ = a × (1)² + b × (1)³ = a – b = 5 But we don't know what a + b equals to in this case (if x = 1).
Therefore statement (1) by itself is NOT sufficient.


Top 


questioner

Post subject: Re: math (test 1, question 28): algebra. Posted: Mon Feb 28, 2011 2:35 pm 

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

By default, all the numbers in GMAT are real. Therefore, both x = 1 and x = are valid solutions for (2). While 1 yields the right answer, does not. In conclusion, we have to use Statement 1 to make sure, that x = 1, not .


Top 


Gennadiy

Post subject: Re: math (test 1, question 28): algebra. Posted: Mon Feb 28, 2011 2:44 pm 

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

The values and 1 are equal.
The second statement yields x³ = 1. This equation has only one solution: x = ³√1 = 1.


Top 


questioner

Post subject: Re: math (test 1, question 28): algebra. Posted: Thu Sep 01, 2011 12:55 pm 

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

In case of 1, if x is 1, then we get a – b = 5 and when x is 1, we get a + b =5. Since we get only 1 value of a + b from (1), I feel the answer to this should be D.


Top 


Gennadiy

Post subject: Re: math (test 1, question 28): algebra. Posted: Thu Sep 01, 2011 1:21 pm 

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


Top 


questioner

Post subject: Re: math (test 1, question 28): algebra. Posted: Tue Sep 20, 2011 5:23 pm 

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

a, b and x could be rational numbers, it is not mentioned that they are integers. Hence, the inference that x could only be 1 or 1 is wrong; e.g. x = 1/2, a = 4 and b = 32.


Top 


Gennadiy

Post subject: Re: math (test 1, question 28): algebra. Posted: Tue Sep 20, 2011 5:32 pm 

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


Top 


cjuarezh

Post subject: Re: GMAT Algebra (Data Sufficiency) Posted: Tue Aug 07, 2012 2:36 am 

Joined: Tue Aug 07, 2012 2:26 am Posts: 1

I desagree with the given answer, we have:
If ax² + bx³ = 5, where a and b are nonzero numbers, what is the value of a + b? (1) ax² = a (2) bx³ = b
I agree that with (1) we cannot answer the question, but (2) isn't enough either.
number 2 says that x³ = 1 (with b nonzero), which has 3 solutions, not 1:
x³  1 = 0 (x – 1) × (x² + x + 1)=0 Either (x – 1) = 0 or (x² + x + 1) = 0. Only x – 1 = 0 gives x = 1. But the 3 solutions are:
1, 1/2 + (√3 / 2) × i, 1/2 – (√3 / 2) × i
The second and third solution, when squared, don't equal 1, so we need the first statement to know that x can't take these values. Therefore, the answer is C and not B.


Top 


Gennadiy

Post subject: Re: GMAT Algebra (Data Sufficiency) Posted: Wed Aug 08, 2012 1:20 pm 

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


Top 



Page 1 of 1

[ 10 posts ] 

Who is online 
Users browsing this forum: No registered users and 4 guests 

You cannot post new topics in this forum You cannot reply to topics in this forum You cannot edit your posts in this forum You cannot delete your posts in this forum You cannot post attachments in this forum

