我该如何调整案件？

Question

行距太小。如何调整？

\begin{equation}\tag{12}
  \left\{
    \begin{array}{ll}
      U_{1}(x) = \sum_{|n| \leq N} A_{n}e^{i\alpha_{n}x} + \sum^{2}_{l
      = 1}a_{l} \frac{x^{l}}{l!} \\
      U^{'}_{1}(x) = \sum_{|n| \leq N}
      A_{n}i\alpha_{n}e^{i\alpha_{n}x} + \sum^{2}_{l = 1}a_{l}
      \frac{x^{l - 1}}{(l - 1)!}
    \end{array}
  \right. \texttt{where $\alpha_{n}$ = 2n$\pi{}$}
\end{equation}

Answer 1

两个解决方案的代码更简单：一个dcases来自 的环境（displaystyle cases）mathtools，另一个来自empheq环境。不加载amsmath：empheq加载mathtools，这会加载它。

\documentclass{article}
\usepackage{empheq}

\begin{document}

\begin{equation}
  \begin{dcases}
     U_1(x) = \sum_{|n| \leq N}A_n e^{i\alpha_n x} + \sum^2_{l=1}a_l \frac{x^l}{l!} \\
    U'_1(x) = \sum_{|n| \leq N}A_n i\alpha_n e^{i\alpha_n x} + \sum^2_{l=1}a_l \frac{x^{l - 1}}{(l - 1)!}
  \end{dcases}
  \quad\text{where}\quad \alpha_n = 2n\pi
\end{equation}

\begin{empheq}[left=\empheqlbrace]{equation}
  \begin{aligned}
     U_1(x) & = \sum_{|n| \leq N}A_n e^{i\alpha_n x} + \sum^2_{l=1}a_l \frac{x^l}{l!} \\%[2\jot]
    U'_1(x) & = \sum_{|n| \leq N}A_n i\alpha_n e^{i\alpha_n x} + \sum^2_{l=1}a_l \frac{x^{l - 1}}{(l - 1)!}
  \end{aligned}
  \quad \text{where}\quad \alpha_n = 2n\pi
\end{empheq}

\end{document} 

Answer 2

这里有一些选项，包括添加\displaystyle垂直拉伸案例：

\documentclass{article}

\usepackage{amsmath}

\begin{document}

\begin{equation}
  \left\{
  \begin{array}{ l l }
     U_1(x) = \sum_{|n| \leq N}A_n e^{i\alpha_n x} + \sum^2_{l=1}a_l \frac{x^l}{l!} \\
    U'_1(x) = \sum_{|n| \leq N}A_n i\alpha_n e^{i\alpha_n x} + \sum^2_{l=1}a_l \frac{x^{l - 1}}{(l - 1)!}
  \end{array}
  \right. \text{where $\alpha_n = 2n\pi$}
\end{equation}

\begin{equation}
  \left\{\setlength{\arraycolsep}{0pt}
  \begin{array}{ r l @{\quad} l }
     U_1(x) &{}= \sum_{|n| \leq N}A_n e^{i\alpha_n x} + \sum^2_{l=1}a_l \frac{x^l}{l!} \\[2\jot]
    U'_1(x) &{}= \sum_{|n| \leq N}A_n i\alpha_n e^{i\alpha_n x} + \sum^2_{l=1}a_l \frac{x^{l - 1}}{(l - 1)!}
  \end{array}
  \right. \text{where $\alpha_n = 2n\pi$}
\end{equation}

\begin{equation}
  \left\{\setlength{\arraycolsep}{0pt}
  \begin{array}{ r l @{\quad} l }
     U_1(x) &{}= \displaystyle\sum_{|n| \leq N}A_n e^{i\alpha_n x} + \sum^2_{l=1}a_l \frac{x^l}{l!} \\[\jot]
    U'_1(x) &{}= \displaystyle\sum_{|n| \leq N}A_n i\alpha_n e^{i\alpha_n x} + \sum^2_{l=1}a_l \frac{x^{l - 1}}{(l - 1)!}
  \end{array}
  \right. \text{where $\alpha_n = 2n\pi$}
\end{equation}

\end{document}

最后，由于您使用array设置案例，因此您可以使用以下方法调整垂直间距表格中的列和行填充。