Math & Science ⇒ Reference to Equation fails

[GustavMJ]

Hi!

I'm totally new to both this forum and to LaTeX and have a question regarding equation numbering. I have used the command \label inside one of my equations, but later when I use the \eqref I get an error saying:

Undefined control sequence [...] The control sequence at the end of the top line of your error message was never \def'ed. If you have misspelled it (e.g., `\hobx'), type `I' and the correct spelling (e.g., `I\hbox').

I have checked so that the code is correctly spelled and everything looks fine, but still I get this error. My code looks like this.

Code: Select all

\begin{equation}
\label{eq:malus}
I_\theta = I_0 {\cos}^2 \theta
\end{equation}

Followed by \eqref{eq:malus}. I'm using the following packages and document class.

Code: Select all

\documentclass[11pt]{article}
\usepackage[left=1in, right=1in, top=1in, bottom=1in]{geometry}
\usepackage{graphicx}
\usepackage[T1]{fontenc}
\usepackage{lmodern}
\usepackage[utf8]{inputenc}

Thanks!

[svend_tveskaeg]

You need to load the amsmath package.

Code: Select all

\documentclass{article}

\usepackage{amsmath}

\begin{document}

\begin{equation}
 \label{eq:malus}
  I_\theta = I_0\cos^2\theta
\end{equation}

Reference: \eqref{eq:malus}

\end{document}

[localghost]

The \eqref command is provided by the amsmath package which you missed to load.


Further reading:

• The »Math mode« document

Best regards and welcome to the board
Thorsten

[GustavMJ]

Ah, thanks :]