\documentclass[a0paper]{a0poster}
\usepackage{tikz,pgf}
\usetikzlibrary{shapes,snakes}
\usepackage{graphics, times}
\usepackage{eso-pic}
\usepackage[absolute]{textpos}
\usepackage{amsmath,amssymb}
\TPGrid[40mm,40mm]{23}{12}   



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\begin{document}




\begin{textblock}{15}(15,1.23)
\tikzstyle{mybox} = [draw=red, fill=red!40!yellow!50, very thick,
    rectangle, rounded corners, inner sep=10pt, inner ysep=20pt]
\tikzstyle{fancytitle} =[very thick,draw=black,fill=red, text=blue, inner ysep=20pt]
\begin{tikzpicture}
\node [mybox] (box){\LARGE
    \begin{minipage}{0.50\textwidth}
        \begin{align}
            \dot{n} &= u\cos\psi -v\sin\psi \\
            \dot{e} &= u\sin\psi + v\cos\psi
        \end{align}
      \end{minipage}
};
\node[fancytitle,above=0pt] at (box.north) {

\begin{minipage}{0.50\textwidth}
\LARGE tittle 
    \end{minipage}
};
\end{tikzpicture}%

\end{textblock}
\begin{textblock}{15}(15,5.23)
\begin{tikzpicture}[rounded corners,ultra thick]
\shade[top color=yellow,bottom color=black] (0,0) rectangle +(20,11);
\end{tikzpicture}
\end{textblock}
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\begin{tikzpicture}[rounded corners,ultra thick]
\shade[top color=yellow,bottom color=black] (0,0) rectangle +(20,11);
\end{tikzpicture}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\end{document}
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