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by badri_1 » Thu Feb 23, 2012 8:32 pm
Sorry to ask this once again. But still trying to understand why B is the correct answer.
If a=b=1 and n=0 then the second option becomes 1+1 which is divisible by a+b(2).
Doesn't this mean we get both ZERO and all ODD values not satisfying the second option. Hence unable to answer the question: is n odd?
Can someone clarify.
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by rohan.saha86 » Wed Feb 29, 2012 10:33 pm
for a^n-b^n, you can pick n=2 or 3 and it is divisible in either case so 1) is not sufficient
For a^n+b^n, again piking the values n=2 or 3, we can see that for n=2 it is definitiely not divisible by a+b
Hence, 2.) is sufficient, and answer is B)
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by krusta80 » Thu Mar 01, 2012 12:59 pm
harsh.champ wrote:Is n odd ?

1. a^n - b^n is divisible by a - b
2. a^n + b^n is not divisible by a + b
(1) a^n - b^n is divisible by a - b

This works for n = 1 -> a-b mod a-b = 0
This also works for any even n >= 2 -> a^2-b^2 -> (a-b)(a+b) mod (a-b) = 0

INSUFF


(2) a^n + b^n is NOT divisible by a+b

SUFF

B
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by fangtray » Sun Apr 01, 2012 5:21 am
krusta80 wrote:
harsh.champ wrote:Is n odd ?

1. a^n - b^n is divisible by a - b
2. a^n + b^n is not divisible by a + b
(1) a^n - b^n is divisible by a - b

This works for n = 1 -> a-b mod a-b = 0
This also works for any even n >= 2 -> a^2-b^2 -> (a-b)(a+b) mod (a-b) = 0

INSUFF


(2) a^n + b^n is NOT divisible by a+b

SUFF

B
ian gave an example where a^n +b^n is divisible by a+b. for example, n=2 a=20 b=5. what am i missing here? how come its still b?>
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by fangtray » Sun Apr 01, 2012 5:24 am
krusta80 wrote:
harsh.champ wrote:Is n odd ?

1. a^n - b^n is divisible by a - b
2. a^n + b^n is not divisible by a + b
(1) a^n - b^n is divisible by a - b

This works for n = 1 -> a-b mod a-b = 0
This also works for any even n >= 2 -> a^2-b^2 -> (a-b)(a+b) mod (a-b) = 0

INSUFF


(2) a^n + b^n is NOT divisible by a+b

SUFF

B
ian gave an example where a^n +b^n is divisible by a+b. for example, n=2 a=20 b=5. what am i missing here? how come its still b?>
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by fangtray » Sun Apr 01, 2012 5:27 am
krusta80 wrote:
harsh.champ wrote:Is n odd ?

1. a^n - b^n is divisible by a - b
2. a^n + b^n is not divisible by a + b
(1) a^n - b^n is divisible by a - b

This works for n = 1 -> a-b mod a-b = 0
This also works for any even n >= 2 -> a^2-b^2 -> (a-b)(a+b) mod (a-b) = 0

INSUFF


(2) a^n + b^n is NOT divisible by a+b

SUFF

B
ian gave an example where a^n +b^n is divisible by a+b. for example, n=2 a=20 b=5. what am i missing here? how come its still b?>
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by adivjay » Thu Apr 05, 2012 8:23 pm
Folks,

I have 3 comments to the posted question:
1) the answer is definitely not A, B, or D. In fact, i think the answer is E.
2) i don't think you can apply any general rule without specifying restrictions on the values of a, b, and n. The answer highly depends on the values of a, b, and n. If you consider various possibilities for these variables (e.g., a=b, a =odd and b=even, one of them negative and another positive, n being negative), you will find that the answer is not concrete (can be yes or no)
3) for a question such as this one, i would simply guess and move on. It is not worth spending the time during the test to examine various scenarios even though it may be highly tempting to get the right answer. Any extra time you spend to solve such a question is less time for other questions and the chances of getting penalized for not completing the section increases. GMAT is not about only being right, but also being smart. Be strategic.
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by krusta80 » Fri Apr 06, 2012 2:56 am
fangtray wrote:
krusta80 wrote:
harsh.champ wrote:Is n odd ?

1. a^n - b^n is divisible by a - b
2. a^n + b^n is not divisible by a + b
(1) a^n - b^n is divisible by a - b

This works for n = 1 -> a-b mod a-b = 0
This also works for any even n >= 2 -> a^2-b^2 -> (a-b)(a+b) mod (a-b) = 0

INSUFF


(2) a^n + b^n is NOT divisible by a+b

SUFF

B
Can someone verify whether the original question places a non-negative restriction on n?
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by sanju09 » Fri Apr 06, 2012 3:31 am
krusta80 wrote:
fangtray wrote:
krusta80 wrote:
harsh.champ wrote:Is n odd ?

1. a^n - b^n is divisible by a - b
2. a^n + b^n is not divisible by a + b
(1) a^n - b^n is divisible by a - b

This works for n = 1 -> a-b mod a-b = 0
This also works for any even n >= 2 -> a^2-b^2 -> (a-b)(a+b) mod (a-b) = 0

INSUFF


(2) a^n + b^n is NOT divisible by a+b

SUFF

B
Can someone verify whether the original question places a non-negative restriction on n?
The original question should have placed the non-negativity constraint for n, otherwise this would prove to be fairly unGMATtish! (new term)
The mind is everything. What you think you become. -Lord Buddha



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by sanju09 » Fri Apr 13, 2012 12:51 am
fangtray wrote:
krusta80 wrote:
harsh.champ wrote:Is n odd ?

1. a^n - b^n is divisible by a - b
2. a^n + b^n is not divisible by a + b
(1) a^n - b^n is divisible by a - b

This works for n = 1 -> a-b mod a-b = 0
This also works for any even n >= 2 -> a^2-b^2 -> (a-b)(a+b) mod (a-b) = 0

INSUFF


(2) a^n + b^n is NOT divisible by a+b

SUFF

B
ian gave an example where a^n +b^n is divisible by a+b. for example, n=2 a=20 b=5. what am i missing here? how come its still b?>
a^n + b^n is divisible by a + b only when n is an odd positive integer, and a and/or b are non zero numbers. You cannot take n = 2 to prove this point, fangtray.
The mind is everything. What you think you become. -Lord Buddha



Sanjeev K Saxena
Quantitative Instructor
The Princeton Review - Manya Abroad
Lucknow-226001

www.manyagroup.com
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by Ganesh hatwar » Mon Jul 23, 2012 5:13 am
harsh.champ wrote:Is n odd ?

1. a^n - b^n is divisible by a - b
2. a^n + b^n is not divisible by a + b
E bcoz i am not getting !!
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by surmilsehgal » Wed Aug 01, 2012 11:38 pm
the answer is B
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by rahul.neemuch » Wed Sep 26, 2012 1:18 am
Hi guys
Pls reply to my query.
Why nt the ans is E.

a n b are real nos. If we take a=3/2, b=1/2 then even if n is odd a^n+b^n is not divisible by a+b.
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by rahul.neemuch » Wed Sep 26, 2012 1:19 am
Hi guys
Pls reply to my query.
Why nt the ans is E.

a n b are real nos. If we take a=3/2, b=1/2 then even if n is odd a^n+b^n is not divisible by a+b.
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by rahul.neemuch » Wed Sep 26, 2012 1:19 am
Hi guys
Pls reply to my query.
Why nt the ans is E.

a n b are real nos. If we take a=3/2, b=1/2 then even if n is odd a^n+b^n is not divisible by a+b.
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