Equation in a yellow box

Question

I'm trying to insert a numbered equation inside a yellow box. The problem is that the yellow box extends across the entire line. How can I avoid it?

\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage[most]{tcolorbox}

\newtcolorbox{mybox}{
enhanced,
boxrule=0pt,frame hidden,
colback=yellow!40!white,
sharp corners
}

\begin{document}
\section{Introduction}
\begin{mybox}
\begin{equation}
a = b
\end{equation}
\end{mybox}

\end{document}

Answer 1

If all you need to achieve is to place a formula in a colored box, employing the machinery of the tcolorbox may be overkill. For sure, using the \colorbox macro of the xcolor package would require far less overhead.

\documentclass{article}
\usepackage{xcolor} % for '\colorbox' macro
\begin{document}
\section{Introduction}

\begin{equation}
\colorbox{yellow!40!white}{$\displaystyle ~a = b~\vphantom{\int}$}
\end{equation}

\end{document}

(I trust you can figure out what \vphantom{\int} and ~ are doing. If not, please let me know and I'll provide an explanation.)

Answer 2

With empheq package:

\documentclass{article}
\usepackage{empheq}
\usepackage{xcolor}


\begin{document}
{\setlength\fboxsep{3ex}
\begin{empheq}[box=\colorbox{yellow}]{equation}
a = b 
\end{empheq}
}
\end{document}

Note: Size of yellow box is determined by border around equation. It is defined by fboxsep. You can determine it locally as is done in above MWE or globally, if you move \setlength\fboxsep{3ex} to the document preamble. You may find other fboxsep size more appropriate, for example 2ex, which gives:

Answer 3

Probably the output is closer to the expected one, when using \tcbhighmath:

\documentclass{article}
\usepackage[most]{tcolorbox}

\tcbset{highlight math/.append style={boxrule=0pt,
                                      frame hidden,
                                      colback=yellow!40!white,
                                      sharp corners}}
\begin{document}
\section{Introduction}
\begin{equation}
\tcbhighmath{a=b}
\end{equation}
\end{document}