![乳胶中的数学表达式](https://linux22.com/image/373981/%E4%B9%B3%E8%83%B6%E4%B8%AD%E7%9A%84%E6%95%B0%E5%AD%A6%E8%A1%A8%E8%BE%BE%E5%BC%8F.png)
答案1
以下是一种方法:
\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage{fourier}
\usepackage[thin, , thinc]{esdiff}
\begin{document}
\[
\dot\sigma = \diffp{\sigma}{{\mathbf x}}\overbrace{\dot{\mathbf x}}^{\diff{\mathbf x}{t}}\bigl(\mkern1.5mu\overbrace{f(\mathbf x, t) + B(t, \mathbf x)\mathbf u}^{\dot{\mathbf x}}\mkern1.5mu \bigr)
\]
\end{document}