[Tex/LaTex] ! Missing } inserted. }

Question

\begin{eqnarray}[\{]
$$\frac{\partial{x_i}}{\partial{q_j}} = \sum_{j = 1}^{n}\frac{\partial{x_i}}{\partial{q_j}}  $$ \\
$$\frac{\partial{y_i}}{\partial{q_j}} = \sum_{j = 1}^{n}\frac{\partial{y_i}}{\partial{q_j}}  $$ \\
$$\frac{\partial{z_i}}{\partial{q_j}} = \sum_{j = 1}^{n}\frac{\partial{z_i}}{\partial{q_j}}  $$
\end{eqnarray}

I don't see the fault here. Could you guys please help me ??

Answer 1

You shouldn't use eqnarray, as it leads to bad spacings. Use the amsmath environments instead. I suppose you're after this:

\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}

\usepackage{empheq}
\usepackage{esdiff}

\begin{document}

\begin{empheq}[left = \empheqlbrace\;]{align}
\diffp{x_i}{{q_j}} & = \smash[t]{\sum_{j = 1}^{n}}\diffp{x_i}{{q_j}} \\
\diffp{y_i}{{q_j}} & = \sum_{j = 1}^{n}\diffp{y_i}{{q_j}} \\
\diffp{z_i}{{q_j}} & = \smash[b]{\sum_{j = 1}^{n}}\diffp{z_i}{{q_j}}
\end{empheq}

\end{document}