使用 \begin{cases} 对齐方程

Question

我希望底部方程的开头与顶部方程的开头对齐。同样，我希望所有“for”都对齐，这样底部的也与顶部的对齐。最好的方法是什么？

提前致谢！

\documentclass[a4paper,12pt,default,numbered,print,index]{article}
\usepackage{lipsum}
\usepackage{enumitem}
\usepackage{graphicx} % Required for the inclusion of images
\usepackage{setspace} % for use of \singlespacing and \doublespacing
\usepackage{pdfpages}
\usepackage{cite}
\usepackage[section]{placeins}
\usepackage{comment}
\usepackage{siunitx}
\usepackage{color}
\usepackage{ragged2e}
\usepackage{esvect}
\usepackage{mathtools}
\usepackage{lscape}
\usepackage{tabularx}
\usepackage{multirow}
\usepackage{array}
\usepackage{soul}
\usepackage{bm}
\usepackage{url}
\usepackage{xparse}
\usepackage{hyperref}

\begin{document}

\begin{equation}
\frac{p'}{\gamma \bar{p}}\Bigg|_d(x,t) \approx 
\begin{cases} 
      (P_d^+ + P_d^-R_1e^{-2\alpha(L_1 + x)})(1 + R_2e^{-2\alpha(L_2 - x)})\\ \times \dfrac{1- (R_1R_2e^{-2\alpha L})^{\lfloor (t + T)/T\rfloor}}{1-R_1R_2e^{-2\alpha L}}
      & \text{for}\enspace t \in [0,t_p]
      \\
      2P_D(R_1R_2e^{-2\alpha L})^{\lfloor (t - t_p + T)/T\rfloor}
      & \text{for}\enspace t \in ] t_p, +\infty[.
\end{cases}
\end{equation}

\begin{equation}
\frac{p'}{\gamma \bar{p}}\Bigg|_i(x,t) \approx
\begin{cases} 
      P_i^+(1 + ^-R_1e^{-2\alpha(L_1 + x)}) \\ \times \dfrac{1- (R_1R_2e^{-2\alpha L})^{\lfloor (t - \tau_c + T)/T\rfloor}}{1-R_1R_2e^{-2\alpha L}}
      & \text{for}\enspace t \in [0,t_p]
      \\
      2P_I(R_1R_2e^{-2\alpha L})^{\lfloor (t - t_p - \tau_c + T)/T\rfloor}
      & \text{for}\enspace t \in ] t_p, +\infty[.
\end{cases}
\end{equation}
\end{document}

Answer 1

由于第一行太长，因此将两个equation环境组合成一个align并使用。\mathrlap

array请注意，如果您加载了，则无需加载tabularx。

\documentclass[a4paper,12pt,default,numbered,print,index]{article}
\usepackage{lipsum}
\usepackage{enumitem}
\usepackage{graphicx} % Required for the inclusion of images
\usepackage{setspace} % for use of \singlespacing and \doublespacing
\usepackage{pdfpages}
\usepackage{cite}
\usepackage[section]{placeins}
\usepackage{comment}
\usepackage{siunitx}
\usepackage{xcolor}
\usepackage{ragged2e}
\usepackage{esvect}
\usepackage{mathtools, bm}
\usepackage{lscape}
\usepackage{tabularx}
\usepackage{multirow}
\usepackage{soul}
\usepackage{url}
\usepackage{xparse}
\usepackage{hyperref}

\begin{document}

\setcounter{section}{2}
\renewcommand\theequation{\thesection.\arabic{equation}}
\setcounter{equation}{39}
\begin{align}
\frac{p'}{\gamma \bar{p}}\Bigg|_d(x,t) & \approx
\begin{cases}
      (P_d^+ + P_d^-R_1e^{-2\alpha(L_1 + x)})(1 + R_2\mathrlap{e^{-2\alpha(L_2 - x)})}\\ \times \dfrac{1- (R_1R_2e^{-2\alpha L})^{\lfloor (t + T)/T\rfloor}}{1-R_1R_2e^{-2\alpha L}}
      & \text{for}\enspace t \in [0,t_p]
      \\
      2P_D(R_1R_2e^{-2\alpha L})^{\lfloor (t - t_p + T)/T\rfloor}
      & \text{for}\enspace t \in {] t_p, +\infty[}.
\end{cases} \\[1.5ex]
\frac{p'}{\gamma \bar{p}}\Bigg|_i(x,t) & \approx
\begin{cases}
      P_i^+(1 + ^-R_1e^{-2\alpha(L_1 + x)}) \\ \times \dfrac{1- (R_1R_2e^{-2\alpha L})^{\lfloor (t - \tau_c + T)/T\rfloor}}{1-R_1R_2e^{-2\alpha L}}
      & \text{for}\enspace t \in [0,t_p]
      \\
      2P_I(R_1R_2e^{-2\alpha L})^{\lfloor (t - t_p - \tau_c + T)/T\rfloor}
      & \text{for}\enspace t \in {] t_p, +\infty[}.
\end{cases}
\end{align}

\end{document}