Using \eqno in math mode

Question

does anyone know, why do I keep getting this error message?

!You cant use '\eqno' in math mode.

..and it's pointing at this line in my code:

\left(\sqrt[5]{x^4}\right)’ &=& \left(x^{[\frac{4}{5}\]}\right)’ &=& [\frac{4}{5}\]x^{[-\frac{1}{5}\]} &=& [\frac{4}{5\sqrt[5]{x}\}] \label{rovnice3} \\

here's the whole array:

\begin{eqnarray}
\int_a^b f(x)\,\mathrm{d}x &=& -\int\limits_b^a f(x)\,\mathrm{d}x \label{rovnice2}\\
\left(\sqrt[5]{x^4}\right)’ &=& \left(x^{[\frac{4}{5}\]}\right)’ &=& [\frac{4}{5}\]x^{[-\frac{1}{5}\]} &=& [\frac{4}{5\sqrt[5]{x}\}] \label{rovnice3} \\    \overline{\overline{A \lor B}} &=& \overline{\overline{A} \land \overline{B}\label{rovnice4} \end{eqnarray}

Thanks in advance!

Answer 1

There are some issues:

• \] instead of ]. \] is the end of an unnumbered equation and interferes with the current environment.

• \} instead of }, also a missing }, both causing errors, because the group/argument braces are not properly nested.

• ` instead of '. The short form of ^{\prime} is the ASCII apostrophe (decimal 37, hexadecimal 0x27).

• eqnarray only supports two &. Only the second equation line has more than one relational symbol, thus at most the first equal sign needs to be aligned.

The following example tries to fix the various issues. Also a variant with align of amsmath is added that also removes some unnecessary delimiters:

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{eqnarray}
  \int_a^b f(x)\,\mathrm{d}x
  &=&
  -\int\limits_b^a f(x)\,\mathrm{d}x
  \label{rovnice2}
\\
  \left(\sqrt[5]{x^4}\right)'
  &=&
  \left(x^{\left[\frac{4}{5}\right]}\right)'
  =
  \left[\frac{4}{5}\right]x^{\left[-\frac{1}{5}\right]}
  =
  \left[\frac{4}{5\sqrt[5]{x}}\right]
  \label{rovnice3}
\\
  \overline{\overline{A \lor B}}
  &=&
  \overline{\overline{A} \land \overline{B}}
  \label{rovnice4}
\end{eqnarray}

\begin{align}
  \int_a^b f(x)\,\mathrm{d}x
  &=
  -\int\limits_b^a f(x)\,\mathrm{d}x
  \label{rovnice2b}
\\
  \left(\sqrt[5]{x^4}\right)'
  &=
  \left(x^{\left[\frac{4}{5}\right]}\right)'
  =
  \frac{4}{5}x^{-\frac{1}{5}}
  =
  \frac{4}{5\sqrt[5]{x}}
  \label{rovnice3b}
\\
  \overline{\overline{A \lor B}}
  &=
  \overline{\overline{A} \land \overline{B}}
  \label{rovnice4b}
\end{align}
\end{document}

Answer 2

Please always post complete documents but in an eqnarray you can only have two & per line you have

 &=& [\frac{4}{5}\]x^{[-\frac{1}{5}\]} &=& [\fr

so that is four &

But basically you should never use eqnarray always use align and similar environments from amsmath.