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\begin{equation}
= \left(\begin{array} {cccc}
\ cos\theta_i   & -cos\alpha_i \cdot sin\theta_i & sin \alpha_i \cdot sin\theta_i   & a_i \cdot cos\theta_i    \\	 
\ sin \theta_i  & cos \alpha_i \cdot cos\theta_i & -sin\alpha_i \cdot cos\theta_i   & a_i \cdot sin\theta_i    \\
\ 0 	        & sin \alpha_i                   & cos \alpha_i                     & d_i  	                   \\
\ 0	            & 0                              & 0                                & 1                        \\
\end{array} \right) \\ 
\end{equation}

\documentclass[12pt]{report}

\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{mathcomp}
\usepackage[utf8]{inputenc}


\usepackage[pdftex]{graphicx}
\usepackage{subfigure}
\usepackage{geometry}
\geometry{a4paper,top=25mm,left=25mm,right=20mm,bottom=30mm} 

\begin{document}

\begin{equation}
= \left(\begin{array} {cccc}
\ cos\theta_i   & -cos\alpha_i \cdot sin\theta_i & sin \alpha_i \cdot sin\theta_i   & a_i \cdot cos\theta_i    \\	 
\ sin \theta_i  & cos \alpha_i \cdot cos\theta_i & -sin\alpha_i \cdot cos\theta_i   & a_i \cdot sin\theta_i    \\
\ 0 	        & sin \alpha_i                   & cos \alpha_i                     & d_i  	                   \\
\ 0	            & 0                              & 0                                & 1                        \\
\end{array} \right) \\ 
\end{equation}

\end{document}