I would like to use the sum sign in an conditional expression. There is no error message but the Fz is supposed to be under the sum sign but instead its a subscript. Can someone help me solving my problem?

\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}

• Welcome to TEX.SX. Could you please put a compilable minimal working example to reproduce the problem you face?
– Diaa
Sep 4, 2016 at 12:36
• I would see the solution given by Peter for tex.stackexchange.com/questions/3262/… Sep 4, 2016 at 12:38
• Essentially you're trying to achieve a \displaystyle output for something set in\textstyle.
– Werner
Sep 4, 2016 at 13:57

Crystal ball suggests that you might be looking for this:

% 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}


Output: This works:

\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}