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Possible Duplicate:
Division by $0$

I was solving a question for my brother today when i got this doubt, i

I arrived at the answer as $\frac{-1}{0}$

Will the answer be infinity since any finite number divided by zero is infinity or should i write it as - $\frac{-1}{0}$ = - infinity ?

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    It is neither -- division by zero is _undefined_ (it is not "infinity" because infinity is a concept, not a number). So you don't have an answer, but you possibly have an argument that there _is no_ answer to whatever the original question is.2011-12-31
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    An argument that involves division by $0$ is **never** correct, though the intuitive idea behind it can often be turned into a correct argument. You would need to supply the actual problem to get more detail.2011-12-31
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    see http://math.stackexchange.com/questions/26445, http://math.stackexchange.com/search?q=division++zero&submit=search2011-12-31
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    I think he means limits of the form $\frac{-1}{0}$.2011-12-31
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    Once I had a similar confusion: http://math.stackexchange.com/questions/64456/how-to-define-infty2011-12-31

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The expression is not well defined, consider for example

$$\lim_{x \rightarrow \infty}\frac{-1}{\frac{1}{x}}=-\infty$$

whereas

$$\lim_{x \rightarrow \infty}\frac{-1}{\frac{1}{-x}}=+\infty$$

Both expressions have the form $\frac{-1}{0}$. You have to know how the lower sequence approaches 0, it can even be that the limit does not exist.