\colorbox shape – highlight formula

Question

I've seen somewhere such a shape of a highlight.

I am trying to repeat this using xcolor package, but I managed to get only rectangular shape of the box.

\colorbox{red!10}{$\displaystyle\int xe^{-x}dx$}

Answer 1

One can combine tcolorbox and empheq. With \tcbhighmath you can highlight individual subexpressions.

\documentclass{article}
\usepackage[theorems,skins]{tcolorbox}
\usepackage{empheq}
\tcbset{red eqbox/.style={enhanced,top=0.2ex, bottom=0.2ex, 
left=0.1ex,right=0.1ex,
overlay={\fill[red!10] (frame.south west) to[bend left] 
 (frame.north west) --  (frame.north east) to[bend left]
  (frame.south east) -- cycle;},
boxrule=0pt},
blue eqbox/.style={enhanced,top=0.2ex, bottom=0.2ex, 
left=0.1ex,right=0.1ex,
overlay={\fill[blue!10] (frame.south west) to[bend left] 
 (frame.north west) --  (frame.north east) to[bend left]
  (frame.south east) -- cycle;},
boxrule=0pt},
highlight math style=red eqbox}
\newcommand{\diff}{\mathop{}\!\mathrm{d}}
\begin{document}
\begin{empheq}[box=\tcbhighmath]{equation}
\int x \,e^{-x}\,\diff x
\end{empheq}

\begin{align}
\tcbhighmath{\int x \,e^{-x}\,\diff x}~&=
\left.\int x \,e^{-ax}\,\diff x\right|_{a=1}
\notag\\
&=
\left[-\frac{\diff}{\diff a}
\int e^{-ax}\,\diff x\right]_{a=1}
=\left[\frac{\diff}{\diff a}\frac{e^{-ax}}{a}\right]_{a=1}
\notag\\
&=~\tcbhighmath[blue eqbox]{-(1+x)e^{-x}}
\end{align}
\end{document}

You can always change highlight math style to change the appearance.