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I have the function $f(x) = 1 + \frac{12x+4}{\left( x+1 \right)^2} \cdot \left( \frac{12}{12x+4} - \frac{2}{x+1} \right)$ (actually the derivate of another function, but that shouldn't matter). Since $12x+4=0$ for $x=\frac{-1}{3}$ I would have said the function value is undefined at that point - but Wolframalpha says it's 28! Now what I think might have happened is that WA secretly showed me the limit of $x\to\frac{-1}{3}$ and just didn't bother to tell me. Or, am I missing something else?

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    What exactly did you type in WA? Obviously it canceled the $12x+4$!2012-05-05
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    1 + (12x+4)/((x+1)^2) * (12/(12x+4) - 2/(x+1)), x=-1/32012-05-05
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    That's OK. Removable singularities are for removing.2012-05-05
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    The function as you have expressed it is indeed undefined at $x=-\frac{1}{3}$. However, it does have a limit as $x \rightarrow -\frac{1}{3}$, so you can justifiably extend the definition of $f$ to that point.2012-05-05
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    -1 for Wolfram Alpha2012-05-05
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    I find it annoying that these details (if you consider it a detail) are usually glossed over without comment by teachers.2012-05-05
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    Actually, our Prof is very thorough when it comes to those kind of things, I just wasn't sure if it was actually ok to fix the singularity in this case...2012-05-05

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if we consider function without multiplication ,yes it is undefined at $x=-1/3$ ,but we can remove it by multiplication ,so after we execute this operation we get $f(x)=1+12/(x+1)^2-(24*x+8)/(x+1)^3$,at $x=-1/3$ yes $f(-1/3)$=28

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    Can I just do it like that? Granted, the original function is continuous at $-1/3$ so I can probably do that in this case, but are such things ok in general?2012-05-05
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    it depends,as others mentioned ,it is like removable point,like in limits,so you may do it in some cases,but actually i think that it requires some permision before do this2012-05-05
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    i hope it would help you,good lucks2012-05-05