%
% parabel.tex -- template for standalon tikz images
%
% (c) 2021 Prof Dr Andreas Müller, OST Ostschweizer Fachhochschule
%
\documentclass[tikz]{standalone}
\usepackage{amsmath}
\usepackage{times}
\usepackage{txfonts}
\usepackage{pgfplots}
\usepackage{csvsimple}
\usetikzlibrary{arrows,intersections,math,calc}
\begin{document}
\def\skala{1}
\definecolor{darkgreen}{rgb}{0,0.6,0}
\begin{tikzpicture}[>=latex,thick,scale=\skala]

\def\f{2.0}
\def\X{2.7}
\coordinate (F) at (0,\f);

\begin{scope}
	\clip (-6.1,-1) rectangle (6.1,4.6);
	\foreach \x in {-5.5,-5,...,6}{
		\draw[color=gray!30,line width=1pt]
			(\x,4.7) -- (\x,{\x*\x/(4*\f)});
		\draw[color=gray!50,line width=1pt]
			(\x,{\x*\x/(4*\f)}) -- (F);
	}
\end{scope}

\draw[->] (-6.1,0) -- (6.4,0) coordinate[label={$x$}];
\draw[->] (0,-2.3) -- (0,4.8) coordinate[label={right:$y$}];

\begin{scope}
	\clip (-6.05,-1) rectangle (6.05,4.6);
	\draw[color=red,line width=2pt]
		plot[domain=-6.2:6.2,samples=100] ({\x},{\x*\x/(4*\f)});
\end{scope}

\fill[color=darkgreen] (\X,{\X*\X/(4*\f)}) circle[radius=0.08];
\draw[color=darkgreen,line width=1pt] (F) -- (\X,{\X*\X/(4*\f)}) -- (\X,-\f);
\node[color=darkgreen] at (\X,{\X*\X/(4*\f)})
	[below right] {$P{\color{black}\mathstrut=(x,y)}$};

\node[color=darkgreen] at (\X,{0.5*(-\f+\X*\X/(4*\f))})
	[right] {$\overline{Pl}{\color{black}\mathstrut=y+f}$};
\node[color=darkgreen] at ($0.8*(F)+0.2*(\X,{\X*\X/(4*\f)})+(0,-0.2)$)
	[above right]
	{$\overline{PF}{\color{black}\mathstrut=\sqrt{x^2+(y-f)^2}}$};

\node at (F) [above left] {${\color{blue}F}=(0,f)$};
\draw[color=blue,line width=1pt] (-6,-\f) -- (6,-\f);
\fill[color=blue] (F) circle[radius=0.08];
\node[color=blue] at (-4,-\f) [above] {$l$};

\end{tikzpicture}
\end{document}