New environment for boxed equations

Question

I would like to define new environments to highlight certain equations in some way. For example, I would like a highlighted equation to appear in red text or with a box around it. I have the red text version working but the boxed version is giving me a pair of related errors:

Missing } inserted. \begin{boxedeq*}

Extra }, or forgotten $. \end{boxedeq*}

Since there are unmatched braces in the before and after sections of the new environment, it is my understanding that I need to use \bgroup and \egroup instead of { and }. This works for the red text case but the errors indicate that it isn't working for the boxed case (a "missing" } is inserted in the before section and an "extra" } is found in the after section).

Here is the MWE:

\documentclass{article}

\usepackage{amsmath}
\usepackage{graphicx}
\usepackage{color}

% equation/equation* environments highlighted in red
\newenvironment{higheq}{\textcolor{red}\bgroup\begin{equation}}{\end{equation}\egroup}
\newenvironment{higheq*}{\textcolor{red}\bgroup\begin{equation*}}{\end{equation*}\egroup}

% equation/equation* environments highlighted with box
\newenvironment{boxedeq}{\begin{equation}\boxed\bgroup}{\egroup\end{equation}}
\newenvironment{boxedeq*}{\begin{equation*}\boxed\bgroup}{\egroup\end{equation*}}

\begin{document}
\begin{higheq}
\sin^2(x) + \cos^2(x) = 1
\end{higheq}

\begin{equation}
\boxed{\sin^2(x) + \cos^2(x) = 1}
\end{equation}

\begin{boxedeq*}
\sin^2(x) + \cos^2(x) = 1
\end{boxedeq*}

\end{document}

And here is the output:

How do I avoid this error?

Answer 1

Just in case you are interested in fancier boxes, tcolorbox can be used and it nicely interacts with empheq. A little example showing some possibilities:

\documentclass[dvipsnames]{article}
\usepackage{empheq}
\usepackage[many]{tcolorbox}

\tcbset{
  highlight math style={
    enhanced,
    colframe=NavyBlue!70!black,
    colback=NavyBlue!10,
    boxrule=1pt,
  }
}

\newenvironment{boxedeq}
  {\empheq[box=\tcbhighmath]{equation}}
  {\endempheq}
\newenvironment{boxedeq*}
  {\empheq[box=\tcbhighmath]{equation*}}
  {\endempheq}

\begin{document}

\begin{boxedeq}
\sin^2(x) + \cos^2(x) = 1
\end{boxedeq}

\begin{boxedeq*}
\sin^2(x) + \cos^2(x) = 1
\end{boxedeq*}

\tcbset{
  highlight math style={
    enhanced,
    colframe=Maroon,
    colback=Peach!10,
    arc=4pt,
    boxrule=1pt,
  drop fuzzy shadow
  }
}

\begin{boxedeq}
\sin^2(x) + \cos^2(x) = 1
\end{boxedeq}

\begin{boxedeq*}
\sin^2(x) + \cos^2(x) = 1
\end{boxedeq*}

\tcbset{
  highlight math style={
    enhanced,
    colframe=black,
    colback=white,
    arc=0pt,
    boxrule=1pt,
  }
}

\begin{boxedeq}
\sin^2(x) + \cos^2(x) = 1
\end{boxedeq}

\begin{boxedeq*}
\sin^2(x) + \cos^2(x) = 1
\end{boxedeq*}

\end{document}

Answer 2

Saying \boxed\bgroup<tokens>\egroup is not the same as \boxed{<tokens>}. You can't use \bgroup as a substitute for an argument delimiter.

You accomplish your need with the empheq package.

\documentclass{article}
\usepackage{empheq}

\newenvironment{boxedeq}
  {\empheq[box=\fbox]{equation}}
  {\endempheq}
\newenvironment{boxedeq*}
  {\empheq[box=\fbox]{equation*}}
  {\endempheq}

\begin{document}

\begin{boxedeq}
\sin^2(x) + \cos^2(x) = 1
\end{boxedeq}

\begin{boxedeq*}
\sin^2(x) + \cos^2(x) = 1
\end{boxedeq*}

\end{document}