\documentclass[a4paper]{article}
\usepackage{xcolor}
\begin{document}
\begin{eqnarray*}
f (z)
 & = & a f_{1} (z) \\
\end{eqnarray*}
\newpage
\begin{eqnarray*}
f (z)
 & = & a f_{1} (z) \\
& = & a (u_{1} + iv_{1}) \\
& = & a u_{1} + iav_{1} \\
& = & \underbrace{au_{1}}_{u} + i \underbrace{av_{1}}_{v} \\
\end{eqnarray*}
\newpage
\begin{eqnarray*}
\frac{\partial u}{\partial x}
 & = & \frac{\partial}{\partial x} (a u_{1}) \\
& = & a \, \frac{\partial u_{1}}{\partial x} \\
\end{eqnarray*}
\newpage
\begin{eqnarray*}
\frac{\partial u}{\partial y}
 & = & \frac{\partial}{\partial y} (a u_{1}) \\
& = & a \, \frac{\partial u_{1}}{\partial y} \\
\end{eqnarray*}
\newpage
\begin{eqnarray*}
\frac{\partial v}{\partial x}
 & = & \frac{\partial}{\partial x} (av_{1}) \\
& = & a \, \frac{\partial v_{1}}{\partial x} \\
\end{eqnarray*}
\newpage
\begin{eqnarray*}
\frac{\partial v}{\partial y}
 & = & \frac{\partial}{\partial y} (av_{1}) \\
& = & a \, \frac{\partial v_{1}}{\partial y} \\
\end{eqnarray*}
\newpage
\begin{eqnarray*}
\frac{\partial u}{\partial x} - \frac{\partial v}{\partial y}
 & = & 0 \quad \wedge \quad \frac{\partial v}{\partial x} + \frac{\partial u}{\partial y} \, \, \, = \, \, \, 0 \\
\end{eqnarray*}
\newpage
\begin{eqnarray*}
\frac{\partial u}{\partial x} - \frac{\partial v}{\partial y}
 & = & a \, \frac{\partial u_{1}}{\partial x} - a \, \frac{\partial v_{1}}{\partial y} \\
& = & a \underbrace{\left ( \frac{\partial u_{1}}{\partial x} - \frac{\partial v_{1}}{\partial y} \right )}_{\mbox{CR1 van } f_{1}} \\
\end{eqnarray*}
\newpage
\begin{eqnarray*}
\frac{\partial u}{\partial y} + \frac{\partial v}{\partial x}
 & = & a \, \frac{\partial u_{1}}{\partial y} + a \, \frac{\partial v_{1}}{\partial x} \\
& = & a \underbrace{\left ( \frac{\partial u_{1}}{\partial y} + \frac{\partial v_{1}}{\partial x} \right )}_{\mbox{CR2 van } f_{1}} \\
\end{eqnarray*}
\newpage
\color{orange}
\begin{eqnarray*}
f (z)
 & = & a f_{1} (z) \\
\end{eqnarray*}
\end{document}
