\documentclass[a4paper]{article}
\begin{document}
\begin{eqnarray*}
f (z)
 & = & \sum_{n = 0}^{n = k} a_{n} z^{n} \\
\end{eqnarray*}
\newpage
\begin{eqnarray*}
f (z)
 & = & \sum_{n = 0}^{n = k} a_{n} z^{n} \\
& = & a_{0} z^{0} + a_{1} z^{1} + a_{2} z^{2} + a_{3} z^{3} + a_{4} z^{4} + \ldots + a_{k} z^{k} \\
\end{eqnarray*}
\end{document}
