条件表达式中的和号

Question

我想在条件表达式中使用和符号。没有错误消息，但 Fz 应该位于和符号下方，但它却是下标。有人能帮我解决我的问题吗？

\begin{equation}  
X = \left\{  
\begin{array}{ll}  
q_{B} = \sum_{Fz}\left(WT_{Fz}  & , \textnormal{ if } q_{B} > 1800 * N_{FS} \\  
q_{S} = \sum_{Fz}\left(WT_{Fz}  & , \textnormal{ if } q_{B} \leq 1800 * N_{FS}\\  
\end{array}  
\right.  
\end{equation}

Answer 1

水晶球暗示你可能正在寻找这个：

% My standard header for TeX.SX answers:
\documentclass[a4paper]{article} % To avoid confusion, let us explicitly 
                                 % declare the paper format.

\usepackage[T1]{fontenc}         % Not always necessary, but recommended.
% End of standard header.  What follows pertains to the problem at hand.

\usepackage{mathtools}



\begin{document}

Some text.
\begin{equation}
    X = \begin{dcases*}  
        q_{B} = \sum_{Fz} WT_{Fz},  & if $q_{B} > 1800 * N_{FS}$; \\  
        q_{S} = \sum_{Fz} WT_{Fz},  & if $q_{B} \leq 1800 * N_{FS}$ \\  
    \end{dcases*}  
\end{equation}

\end{document}

输出：

Answer 2

这有效：

\documentclass{article}

\usepackage{amsmath}

\begin{document}

\begin{equation}  
X = \begin{cases}   
q_{B} = \sum\limits_{Fz} WT_{Fz}  & , \textnormal{ if } q_{B} > 1800 * N_{FS}, \\  
q_{S} = \sum\limits_{Fz} WT_{Fz}  & , \textnormal{ if } q_{B} \leq 1800 * N_{FS}.\\  
\end{cases}  
\end{equation}


\end{document}