如何在对齐的方程式旁边绘制灰色文本框？

Question

如何在等式下方的红色框中执行相同的选项？提前谢谢您！

Answer 1

这符合你的需要吗？

\documentclass{article}
\usepackage{amsmath}
\usepackage{tikz}
\begin{document}
    We will describe\ldots
    \begin{subequations}
    \begin{flalign}
    \tikz \node [fill=gray,fill opacity=.5, text opacity=1, minimum width=15ex]{\tiny Incident wawe};&& E_i & = E_{i0}&\\
    \tikz \node [fill=gray,fill opacity=.5, text opacity=1, minimum width=15ex]{\tiny Reflected wawe};&& E_r & = E_{r0}&\\
    \tikz \node [fill=gray,fill opacity=.5, text opacity=1, minimum width=15ex]{\tiny Transmitted wawe};&& E_t & = E_{t0}&
    \end{flalign}
\end{subequations}
\end{document}