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+\documentclass[a4]{article}
+\usepackage{tikz}
+
+\begin{document}
+
+Draw a box and label the corners and the center:
+
+\begin{tikzpicture}
+	\draw (0,0) -- (0,1) -- (1,1) -- (1,0) -- (0,0);
+	\node [below left] at (0,0) {$a$};
+	\node [above left] at (0,1) {$b$};
+	\node [above right] at (1,1) {$c$};
+	\node [below right] at (1,0) {$d$};
+	\draw [fill] (0.5,0.5) circle [radius=0.5pt];
+	\node [above right] at (0.45,0.45) {\small {$e$}};
+\end{tikzpicture}
+
+\bigskip \hrule \bigskip
+
+Draw some lines:
+
+\bigskip
+
+\begin{tikzpicture}
+	\draw [|-] (0,0) -- (2,0);
+	\draw [|-<] (0,0.5) -- (2,0.5);
+	\draw [|->] (0,1) -- (2,1);
+\end{tikzpicture}
+
+\bigskip \hrule \bigskip
+
+\paragraph{An experiment with polar coordinates}
+
+This does not work: 
+
+\vspace{1.5cm}
+
+\begin{tikzpicture}
+	\tikz \draw [->] (0,0) -- (0:1);
+	\tikz \draw [->] (0,0) -- (72:1);
+	\tikz \draw [->] (0,0) -- (144:1);
+	\tikz \draw [->] (0,0) -- (216:1);
+	\tikz \draw [->] (0,0) -- (288:1);
+\end{tikzpicture}
+
+\bigskip
+
+But this does:
+
+\bigskip
+
+\begin{tikzpicture}
+	\foreach \a in {0, 72,...,288}
+		\draw [->] (0,0) -- (\a:1);
+\end{tikzpicture}
+
+\end{document}
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