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What is the value of {M - N} / {M^2 - N^2}? 1. {M^2 - N^2} =

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What is the value of {M - N} / {M^2 - N^2}? 1. {M^2 - N^2} = [#permalink] New post 26 Oct 2009, 07:56
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A
B
C
D
E

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19% (02:02) correct 81% (00:27) wrong based on 16 sessions
What is the value of {M - N} / {M^2 - N^2}?

1. {M^2 - N^2} = 40
2. [ M + N ] = 10


Pls explain the ans... i thought C but the OA says another .

:x :?:
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Re: DS question : Fractions [#permalink] New post 26 Oct 2009, 08:17
Expert's post
kylexy wrote:
What is the value of {M - N} / {M^2 - N^2}?

1. {M^2 - N^2} = 40
2. [ M + N ] = 10


Pls explain the ans... i thought C but the OA says another .

:x :?:


If it were GMAT question we would most likely have that m\neq{n} and m\neq{-n}, as if m=n or m=-n then the denominator becomes zero and division by zero is undefined. So not a good question.

So if we modify the question and make it GMAT type, we would have:

If m\neq{n} and m\neq{-n}, then what is the value of \frac{m-n}{m^2-n^2}?

\frac{m-n}{m^2-n^2}=\frac{m-n}{(m-n)(m+n)}=\frac{1}{m+n}

(1) m^2-n^2=40 --> Not sufficient.
(2) m+n=10 --> \frac{1}{m+n}=\frac{1}{10}. Sufficient.

Answer: B.

But if we take the question as it is, then the answer would be C, as we won't know for (2) whether the answer is 1/10 or undefined in case m=n.
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Re: DS question : Fractions [#permalink] New post 26 Oct 2009, 08:20
yes, answer should be B.

since (m-n)/m^2-n^2 = 1/(m+n), we only need m+n.
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Re: DS question : Fractions [#permalink] New post 26 Oct 2009, 09:34
Surely B....
M-N/(M+N)(M-N)=1/M+N=1/10
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Re: DS question : Fractions [#permalink] New post 26 Oct 2009, 21:08
Hunt wrote:
Surely B....
M-N/(M+N)(M-N)=1/M+N=1/10



Answer is E...i don understand either!!
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Re: DS question : Fractions [#permalink] New post 27 Oct 2009, 09:54
the above expr can be written as 1/(m+n) & the value of (m+n) is given by statement 2.

I will choose option B
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Re: DS question : Fractions [#permalink] New post 27 Oct 2009, 10:54
kylexy.. it surely can not be E... whats the source......
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Re: DS question : Fractions [#permalink] New post 27 Oct 2009, 11:36
Hunt wrote:
kylexy.. it surely can not be E... whats the source......

its one of the "toughies" from PR
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Pls Help.... [#permalink] New post 27 Sep 2010, 02:13
What is the value of (m-n)/(m*2-n*2) = ?

1.) m*2-n*2 = 40
2.) m+n = 10

I think the answer is B...But the correct answer is E..Please help me explain why????
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Re: Pls Help.... [#permalink] New post 27 Sep 2010, 02:27
sahilkhurana06 wrote:
What is the value of (m-n)/(m*2-n*2) = ?

1.) m*2-n*2 = 40
2.) m+n = 10

I think the answer is B...But the correct answer is E..Please help me explain why????


Moved to DS forum

I take it by m*2-n*2 you mean m^2-n^2 ?

Note that (m-n)/(m^2-n^2) = 1/(m+n) as long as m-n is not equal to 0

1) Not sufficient, as we dont know m+n
2) Would have been sufficient if we knew m-n is not 0.

1+2) Sufficient, as we know m+n and we also know m-n is not zero, as m^2-n^2=(m+n)(m-n)

So answer should be (C)
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Re: Pls Help.... [#permalink] New post 27 Sep 2010, 02:35
Yes..there was some confusion..Actually the answer is C only..Thanks for giving your precious time..
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Re: DS question : Fractions [#permalink] New post 27 Sep 2010, 07:39
What's the final answer, B or C
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Re: DS question : Fractions [#permalink] New post 28 Sep 2010, 13:37
B is the answer

m^2-n^2=(m+n)(m-n) thus, (m-n)/(m+n)(m-n) equals 1/m+n

since we are given that m+n=10 we have sufficient information to answer this question with (2) alone.

B
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Re: DS question : Fractions [#permalink] New post 28 Sep 2010, 13:55
Bruno2000 wrote:
B is the answer

m^2-n^2=(m+n)(m-n) thus, (m-n)/(m+n)(m-n) equals 1/m+n

since we are given that m+n=10 we have sufficient information to answer this question with (2) alone.

B


Nope. (check shrouded's explanation).

(M-N)/(M^2-N^2) = 1/(M+N) only possible if M =/ N or M=/ -N

1. {M^2 - N^2} = 40 => M =/ N
2. [ M + N ] = 10 => M =/ -N

Therefore, we need both. Hence, C
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Re: DS question : Fractions [#permalink] New post 06 Jan 2011, 08:39
I agree on B

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Re: DS question : Fractions   [#permalink] 06 Jan 2011, 08:39
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