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Re: DS question : Fractions [#permalink]
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 .

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 \(\frac{m-n}{m^2-n^2}\)?

(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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