documentclass[border={1mm 1mm 1mm 1mm}]{standalone}
usepackage{tikz}
usetikzlibrary{intersections}

begin{document}
begin{tikzpicture}
defR{2}
defH{4}
defW{2}
draw [red,domain=90:180] plot ({R*cos(x)}, {R*sin(x)});
draw[blue](0,0)rectangle(-R,H);
end{tikzpicture}
begin{tikzpicture}
defR{4}
defH{4}
defW{2.2}
draw [red,domain=90:180,name path=A] plot ({R*cos(x)}, {R*sin(x)});
draw[red](0,0)--(-R,0);
draw[blue,name path=B](0,0)rectangle(-W,H);
path [name intersections={of=A and B,by={E, F}}];
draw[green](0,0)--(F) node[midway,above,sloped]{H};
draw [fill=red,draw=none] (F) circle(1pt);
end{tikzpicture}
end{document}