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