# How to correct errors from Lyx

I have been using Lyx to edit documents, and after changing the Document Class, I Lyx complains of errors when I try to render a preview (PDF) ...

How can I fix this?

UPDATE

I am not certain this the the correct place ... but anyways ... I can't edit the source of Lyx?

\hline
$e^{f(x)}$ & $f'(x)\cdot e^{f(x)}$\tabularnewline
\hline
$ln(f(x))$ &
$\frac{f'(x)}{f(x)}$
\tabularnewline
\hline

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Please add a minimal working example (MWE) (respectively not working) that illustrates your problem. – Stephen Oct 15 '11 at 17:33
>"changing the Document Class": Do you use another document class than before (old one/new one?), or did you change the code of the used class (original code/changed code?)? – Stephen Oct 16 '11 at 13:47
@Stephen, I originally used Lyx with another document class "SGML" (if I'm not wrong) then I changed to another like article. I don't have a problem if I revert my Document Class – Jiew Meng Oct 16 '11 at 21:59
I don't know much about LyX, but $...$ is not allowed in l, c or r columns. In order to get a "big fraction", write $\displaystyle\frac{f'(x)}{f(x)}$ or, if amsmath has been loaded, $\dfrac{f'(x)}{f(x)}$. – egreg Oct 16 '11 at 22:13
I think egreg is right (as usual). When his comment solved your problem, please state so here and ask egreg to re-post his comment as answer, thus that it can be marked as accepted answer (and thereby the question as solved). – Stephen Oct 17 '11 at 11:10

I don't know much about LyX, but $...$ is not allowed in l, c or r columns of a tabular environment.
$\displaystyle\frac{f'(x)}{f(x)}$

$\dfrac{f'(x)}{f(x)}$