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\begin{eqnarray*}
d_{m,n} = \frac{1}{4} \sum_{i = 0}^{7} {\sum_{j = 0}^{7} {a_{ij} \cdot \cos \left( \frac{(2i+1)m\pi}{16}\right) \cdot \cos \left(\frac{(2j+1)n\pi}{16}\right)}}
\end{eqnarray*}

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\documentclass[a4paper,12pt]{scrartcl}
\usepackage{amsmath}
\usepackage{graphicx}
\def\Sum{\ensuremath\mathop{\scalebox{1.5}{$\displaystyle\sum$}}}
\begin{document}
 \begin{align*}
d_{m,n} = \frac{1}{4} \Sum_{i=0}^{7}  {\sum_{j = 0}^{7} {a_{ij} \cdot \cos \left( \frac{(2i+1)m\pi}{16}\right) \cdot \cos \left(\frac{(2j+1)n\pi}{16}\right)}}
\end{align*}
\end{document}