我在原点处画一个 30 度角。组成这个角的两条线段的公共端点是，D其他端点是E和F。我画一个虚线圆弧围绕F。我想使用命令
\draw[dashed,draw=blue!30] (F) -- (F) ++(225:3.75) node [midway,above left]{$5$};
标记圆的半径。（我%在代码中将其放在命令前面。）为什么没有绘制出来？
\documentclass{amsart}
\usepackage{tikz}
\usetikzlibrary{calc,angles,positioning,intersections}
\begin{document}
\begin{tikzpicture}
%E' is a point on the ray from D through E. These commands draw edges $DF$ and
%$E'F$ so that $\angle{\mathit{E'DF}} = 30$. $DF$ is to be drawn at an angle
%120 degrees and DE' is to be drawn at an angle of 150 degrees.
\coordinate (D) at (0,0);
\node (vertex_D) at ($(D) + (-45:7.5pt)$){$D$};
\coordinate (F) at (120:6);
\node (vertex_F) at ($(F) + (90:7.5pt)$){$F$};
\coordinate (E') at (150:9);
\node (vertex_E') at ($(E') + (-90:7.5pt)$){};
\draw (D) -- (F);
\draw[name path=DE'] (D) -- (E');
\path pic[draw, angle radius=10mm,"$30$",angle eccentricity=1.25] {angle = F--D--E'};
%These commands label the length of DF.
\coordinate (DF_midpoint) at ($(D)!0.5!(F)$);
\node (DF_midpoint_label) at ($(DF_midpoint) + (30:7.5pt)$) {8};
%These commands draw the height of the triangle from F to line segment $DE'$.
%The height is labeled $h$.
\coordinate (P) at ($(D)!(F)!(E')$);
\draw [dashed] (F) -- (P) node [midway,above left]{$4$};
\node (point_P) at ($(P) + (-120:7.5pt)$){$P$};
%These commands draw an arc of a circle centered at F with radius 5.
\draw[dashed,draw=blue!30,name path=arc] (F) ++(150:3.75) arc (150:290:3.75);
%\draw[dashed,draw=blue!30] (F) -- (F) ++(225:3.75) node [midway,above left]{$5$};
%The following commands make the right-angle mark.
\coordinate (U) at ($(P)!4mm!45:(D)$);
\draw (U) -- ($(P)!(U)!(D)$);
\draw (U) -- ($(P)!(U)!(F)$);
%These commands label the intersections of the arc with line segment DE'
%intersection-1 and intersection-2.
\draw[name intersections={of= arc and DE'}];
%These commands draw line segments from $F$ to the two intersections on
%line segment DE'.
\draw[red] (F) -- (intersection-1);
\draw[blue] (F) -- (intersection-2);
\end{tikzpicture}
\end{document}
答案1
\tikzlibrary{quotes}缺少了，也许你想要类似的东西
\draw[dashed,draw=blue!30] (F) -- ($(F) +(225:3.75)$) node [midway,above left]{$5$};