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\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 ??
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}
or with use cases package;
\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{cases}
\usepackage{esdiff}
\begin{document}
\begin{numcases}{}
\diffp{x_i}{{q_j}} = \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}} = \sum_{j=1}^{n}\diffp{z_i}{{q_j}}
\end{numcases}
\end{document}
eqnarrayis very wrong ... remove all$and than try again. if you will provide complete but small document we can easily show how to correct type your system of equations. – Zarko Feb 13 '18 at 15:19eqnarrayis considered obsolete. try to use environment fromamsmathpackage. – Zarko Feb 13 '18 at 15:36eqnarrayvsalign. – Werner Feb 13 '18 at 15:37