LaTeX Forum

\documentclass[12pt,a4paper]{scrreprt}
\usepackage{tikz}
\usetikzlibrary{
	quotes,
	angles,
	babel,
	calc
}
\begin{document}
\begin{tikzpicture}
\coordinate (O) at (0,0);
\coordinate (X) at (1,0);
\node[draw,circle,minimum size=4cm](kreis) at (0,0){$\bullet$};
\draw[dashed] (O) -- (-2,2) coordinate(G1);
\draw[dashed] (O) -- (2,2) coordinate(G2);
\end{tikzpicture}
\end{document}

\documentclass[tikz, border=5pt]{standalone}

\usepackage{sansmath}

%\usetikzlibrary{shadings}
\usetikzlibrary{intersections}

\begin{document}
\begin{tikzpicture}[font=\sansmath]
\coordinate (O) at (0,0);

% Hintergrundfarbe der Kugel
%\shade[ball color=blue,opacity=0.2] (0,0) circle [radius=2cm];

% Kegel
\begin{scope}
\def\rx{0.71}  % horizontal radius of the ellipse
\def\ry{0.15}  % vertical radius of the ellipse
\def\z{0.725}  % distance from center of ellipse to origin

\path[name path=ellipse] (0,\z) ellipse ({\rx} and {\ry});
\path[name path=horizontal] (-\rx,\z-\ry*\ry/\z) -- (\rx,\z-\ry*\ry/\z);
\path [name intersections={of = ellipse and horizontal}];

% Radien bis zur Grundfläche des Kegels in der Kugel
\draw[fill=gray!50, gray!50] (intersection-1) -- (0,0) -- (intersection-2) -- cycle;
% Grundfläche des Kegels in der Kugel
\draw[fill=gray!30, densely dashed] (0,\z) ellipse ({\rx} and {\ry}); 
\end{scope}

% Beschriftung des Kegels
\draw (0.25,0.4) -- (0.9,0.1) node at (1.05,0.0) {$q$};

% Kugel
\draw (O) circle [radius=2cm];
% Beschriftung des Mittelpunktes der Kugel
\filldraw (O) circle (1pt) node[below] {$P$}; 

% Radien
\draw[densely dashed] (O) to [edge label=$r$] (-1.33,1.33);
\draw[densely dashed] (O) -- (1.33,1.33);

% Ausschnitt der Kugeloberfläche
\draw[red] (-1.35,1.47) arc [start angle=140, end angle=40, x radius=17.6mm, y radius=14.75mm];
\draw[red, densely dashed] (-1.36,1.46) arc [start angle=170, end angle=10, x radius=13.8mm, y radius=3.6mm];
\draw[red] (-1.29,1.52) arc [start angle=-200, end angle=20, x radius=13.75mm, y radius=3.15mm];

% Beschriftung des Ausschnitts der Kugeloberfläche
\draw (-1.2,2.2) -- (-0.53,1.83) node at (-1.37,2.37) {$A$};

\end{tikzpicture}
\end{document}