\documentclass[a4paper]{article}
\begin{document}
\begin{eqnarray*}
\mbox{(1)} \qquad f (x)
 & = & \sum_{i = 0}^{i = \infty} a_{i} x^{i} \\
\end{eqnarray*}
\newpage
\begin{eqnarray*}
\mbox{(2)} \qquad f (x)
 & = & g (x) h (x) \\
\end{eqnarray*}
\newpage
\begin{eqnarray*}
\mbox{(3)} \qquad g (x)
 & = & \sum_{j = 0}^{j = \infty} b_{j} x^{j} \\
\end{eqnarray*}
\newpage
\begin{eqnarray*}
\mbox{(4)} \qquad f (x)
 & = & g (x) h (x) \\
h (x)
 & = & \frac{f (x)}{g (x)} \\
& = & \frac{\sum_{i = 0}^{i = \infty} a_{i} x^{i}}{\sum_{j = 0}^{j = \infty} b_{j} x^{j}} \\
& = & \frac{a_{0} x^{0} + a_{1} x^{1} + a_{2} x^{2} + a_{3} x^{3} + a_{4} x^{4} + a_{5} x^{5} + \ldots}
{b_{0} x^{0} + b_{1} x^{1} + b_{2} x^{2} + b_{3} x^{3} + b_{4} x^{4} + b_{5} x^{5} + \ldots} \\
& = & \sum_{n = 0}^{n = \infty} c_{n} x^{n} \\
\end{eqnarray*}
\newpage
\begin{eqnarray*}
\mbox{(5)} \qquad c_{n}
 & = & \frac{1}{b_{0}} \left ( a_{n} - \sum_{k = 1}^{k = n} b_{k} c_{n - k} \right ) \\
\end{eqnarray*}
\end{document}
