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\documentclass[tikz]{standalone}
\usetikzlibrary{intersections,decorations.markings}

\begin{document}
\begin{tikzpicture}[
	plot label/.style 2 args={
	    postaction={
            decorate,
            decoration={
                markings,
                mark=at position #1 with \node #2;
            }
        }
    },
    every plot/.style={domain=-95:95},
	x=.05cm, y=.05cm
]
%Raster zeichnen
\draw [color=gray!40, step=5mm] (-110,-110) grid (110,110);
% Achsen zeichnen
\path[->, thick, at end] 
	(-100,0) edge node [right] {$\theta_n$} (100,0) 
	(0,-100) edge node [above] {$\theta_m$} (0,100)
;
% Achsen beschriften
\foreach \coord in {-80, -60, -40, -20, 20, 40, 60, 80}
	\draw 
		(\coord,-1) -- (\coord,1) node [below=4pt] {$\scriptstyle\coord$}
		(-1,\coord) -- (1,\coord) node [left=4pt] {$\scriptstyle\coord$}
	;
%Funktionen zeichnen:
\clip (-108,-108) rectangle (108,108);

\draw[
    name path=A, 
    plot label={.58}{[above]{$\overline{\theta_m}$}}
] plot (\x, 90);

\draw[
    name path = B,
    plot label={.69}{[below]{$\underline{\theta_m}$}}
] plot (\x, -90);

\draw[thick, name path = C] 
    (90,95) -- 
        node [pos=.58, right] {$\overline{\theta_n}$} 
    (90,-95)
;

\draw[thick, name path = D] 
    (-90,95) -- 
        node[pos=.58, left] {$\underline{\theta_n}$} 
    (-90,-95) 
;

\draw[
    name path = E, 
    plot label={.6}{[below right]{$F*{n,m}$}}
] plot (\x, \x - 40);

\draw[
    name path = F,
    plot label={.4}{[above left]{$-F*{n,m}$}}
] plot (\x, \x + 40);

\fill [
    name intersections={of=A and F, by=isAF}, 
    name intersections={of=D and F, by=isDF},
    name intersections={of=B and E, by=isBE},
    name intersections={of=C and E, by=isCE},
    opacity=.5, 
    blue
]
    (isAF) -- (isDF) |- (isBE) -- (isCE) |- cycle
;
\end{tikzpicture} 
\end{document}

\documentclass[a4paper, 12pt]{article}
\usepackage[ngerman]{babel}\usepackage[utf8]{inputenc}
\usepackage{amsfonts}\usepackage{amssymb}\usepackage{amsmath}\usepackage{mathtools}
\usepackage{eurosym}\usepackage{wasysym}\usepackage{graphicx}\usepackage{tikz}\usepackage{adjustbox}\usetikzlibrary{shapes.misc} \usepackage{pgfplots}
\usepgfplotslibrary{fillbetween}
\pgfdeclarelayer{bg}
\pgfsetlayers{bg,main}
\begin{document}
	\begin{tikzpicture}[ cross/.style={draw, cross out,
			minimum size=2*(#1-1pt), inner sep=0pt, outer sep=0pt}, x=.05cm, y=.05cm]
		%Raster zeichnen
		\draw [color=gray!40]  [step=5mm] (-110,-110) grid (110,110);
		% Achsen zeichnen
		\draw[->,thick] (-100,0) -- (100,0) node[right] {$\theta_n$};
		\draw[->,thick] (0,-100) -- (0,100) node[above] {$\theta_m$};
		% Achsen beschriften
		\foreach \x in {-80, -60, -40, -20, 20, 40, 60,80}
		\draw (\x,-1) -- (\x,1) node[below=4pt] {$\scriptstyle\x$};
		\foreach \y in {-80, -60, -40, -20, 20, 40, 60,80}
		\draw (-1,\y) -- (1,\y) node[left=4pt] {$\scriptstyle\y$};
		%Funktionen zeichnen:
		\clip(-108,-108) rectangle (108,108);
		\draw[smooth, domain=-95:95, name path=A] plot (\x, 90) node[above, xshift = -4cm] {$\overline{\theta_m}$};  

		\draw[smooth, domain=-95:95, name path = B] plot (\x, -90) node[below, xshift = -3cm] {$\underline{\theta_m}$};
		\draw[thick, name path = C] (90,95) -- (90,-95) node[right, yshift=4cm] {$\overline{\theta_n}$};  

		\draw[thick, name path = D] (-90,95) -- (-90,-95) node[left, yshift = 4cm] {$\underline{\theta_n}$};  

		\draw[smooth, domain=-95:95, name path = E] plot (\x, \x - 40) node[right, xshift = -4cm, yshift = -4.5cm] {$F{n,m}$};     

		\draw[smooth, domain=-95:95, name path = F] plot (\x, \x + 40) node[left, xshift = -6cm, yshift = -5.5cm] {$-F{n,m}$};
		
	
\end{tikzpicture} 
\end{document}