Quiero trazar la siguiente figura con, por ejemplo, el paquete tikz, pero no sé cómo obtener la forma circular coloreada. (Soy nuevo en esta página, así que me disculpo si mi pregunta es demasiado primitiva).
Respuesta1
Explicaciones mediante comentarios escritos en el código fuente.
\documentclass[border=5mm]{standalone}
\usepackage{tikz}% this package is used to build figures
\usetikzlibrary{angles}% this library allows you to easily draw angles and add text to them. It requires these angles to be defined by 3 points with the operations \node or \coordinate. It is not possible to use coordinates such as (0,0). Thus, I defined 4 points named (a), (b), (o) and (d) when building the first path.
\usetikzlibrary{patterns}% this library allows you to fill the path with a repetitive pattern like a mosaic. I used the pattern `north east lines`.
\usepackage{siunitx}% this package defines the macro \ang{} which allows to correctly display angles in degrees.
\begin{document}
\begin{tikzpicture}
\draw (60:3cm)coordinate(a)--(0,0)coordinate(o)--(-60:3cm)coordinate(b);
% The curve is drawn using 2 bezier curves symmetrical to the horizontal axis. The first starts at `(0,0)` and ends at `(4,0)`. The two points `+(60:2)` and `+(90:2)` are the control points of this Béziers curve. The first control point is `+(60:2)`. The + sign means that its placement is relative to the starting point `(0,0)`. It is placed 2 cm from the starting point at an angle of 60°. The second control point `+(90:2)` is placed relative to the end of the bezier curve `(4,0)`.
\draw[pattern color=red,pattern=north east lines] (0,0)..controls +(60:2) and +(90:2) .. (4,0)coordinate(d)..controls +(-90:2) and +(-60:2)..(0,0);
\draw[->] (0,0)--(3,0);
\draw[dashed] (3,0)--(4.5,0);
\pic [draw,pic text=\ang{60},angle radius=5mm,angle eccentricity=1.6]{angle=d--o--a};
\pic [draw,pic text=\ang{60},angle radius=6mm,angle eccentricity=1.6]{angle=b--o--d};
\end{tikzpicture}
\end{document}