missing $ inserted error for eqnarray

Question

I get the missing $ inserted error(and many other errors like "missing } inserted") for the following code:

    \documentclass[11pt,oneside,onecolumn]{article}

    \usepackage{amsthm}
    \usepackage{verbatim}

    \begin{document}


    \begin{eqnarray}

    f(x)&=&\cos x\\
    f'(x)&=&-\sin x\\
    \int_0^xf(y)\ud y&=&\sin x

    \end{eqnarray}

    \end{document}

I have tried out different changes based on Googling for this error, but nothing seems to help. Any suggestions would be appreciated.

Answer 1

You cannot have blank lines inside the eqnarray environment. Note also the use of \dd from the physics package. Besides, as indicated by @samcarter, align should be preferred.

\documentclass[11pt,oneside,onecolumn]{article}

\usepackage{amsthm}
\usepackage{physics}% for the differential operator \dd


\begin{document}

Using \texttt{eqnarray},
\begin{eqnarray}
  f(x)  &=& \cos x\\
  f'(x) &=& -\sin x\\
  \int_0^x f(y)\dd{y} &=& \sin x                       
\end{eqnarray}
and better, using \texttt{align},
\begin{align}
  f(x)  &= \cos x\\
  f'(x) &= -\sin x\\
  \int_0^x f(y)\dd{y} &= \sin x                       
\end{align}

\end{document}