等式以 ^ 开头

Question

我怎样才能写如下：

\documentclass[12pt,a4paper]{article}
   \usepackage{blindtext}
   \usepackage{indentfirst}
   \usepackage{mathtools}
  \numberwithin{equation}{section}
   \usepackage[hmargin=1in,vmargin=2.5cm]{geometry}
  \usepackage{enumitem}
   \setlength{\parindent}{0.5cm}
  \begin{document}
    \begin{equation} 
    ^{t+\Delta t}\dot{U}=^{t+\Delta t}\dot{U}+\left[\left( 1- \gamma 
    \right)^{t}\ddot{U}+ \gamma ^{t+\delta t}\ddot{U}  \right]           
    \end{equation}
    \end{document}

Answer 1

您可以选择以下之一：

\documentclass[12pt,a4paper]{article}
   \usepackage{blindtext}
   \usepackage{indentfirst}
   \usepackage{mathtools}
  \numberwithin{equation}{section}
   \usepackage[hmargin=1in,vmargin=2.5cm]{geometry}
  \usepackage{enumitem}
   \setlength{\parindent}{0.5cm}

\begin{document}

\section{A demonstrative section}

\begin{align}
   {\vphantom{\Big[}}^{t+\Delta t}\dot{U} & ={\vphantom{\Big[}}^{t+\Delta t}\dot{U}+\left[\left( 1- \gamma
    \right)^{t}\ddot{U}+ \gamma ^{t+\delta t}\ddot{U} \right] \\[3ex] %
    {\vphantom{\bigg[}}^{t+\Delta t}\dot{U} & = {\vphantom{\bigg[}}^{t+\Delta t}\dot{U}+\left[\left( 1- \gamma
    \right)^{t}\ddot{U}+ \gamma ^{t+\delta t}\ddot{U} \right]
\end{align}

\end{document} 

Answer 2

尝试这个：

\documentclass{article}
\begin{document}
\[
\rule{0pt}{15pt}^{t+\Delta t}\dot{U}=\rule{0pt}{15pt}^{t+\Delta t}\dot{U}+
\left[(1-\gamma)^{t}\ddot{U}+\gamma^{t+\Delta t}\ddot{U}\right]\Delta t
\]
\end{document}