\documentclass{article}

\usepackage[utf8]{inputenc}
\usepackage[dvipsnames]{xcolor}
\usepackage{tikz}


% -- Cuerpo del documento
\begin{document}
	Hola mundo.
	
	\section{Ejemplo}
	
	\begin{figure}[h!]
	\centering
	\begin{tikzpicture}
        \draw[thick, OliveGreen, fill=OliveGreen!25] (0, 0) rectangle (4, 4);
        \draw[thick, OliveGreen, fill=White!0] (2, 2) circle (2);
        \draw[semithick, dashed, gray] (2, 2) to (4, 2);
        \node[above] at (3, 2) {$R$};
        \node[below] at (2, 0) {$A = (4 - \pi) R^2$};
	\end{tikzpicture}
	\caption{Área de la región sombreada.}	
	\end{figure}
	
	\section{Ejemplo}
	
	\begin{figure}[h!]
	\centering	
	
	\begin{tikzpicture}
		\draw[thick, Green, fill = Green!15] (0,0) rectangle (5,3);
		\node at (2.5,1.5) {$2x(3x + 7)$};
		\draw[|-|] (5.5,3) to node[fill = white] {$2x$} (5.5,0);
		\draw[|-|] (0,-0.5) to node[fill = white] {$3x + 7$} (5,-0.5);
    \end{tikzpicture}
		
	\caption{Expresión algebraica del área de un rectángulo.}	
	\end{figure}
\end{document}