TikZ 中的路径命令无法正确定位节点

Question

在下图中，我希望将包含标签“P”的节点放置在灰色线段的另一个端点。为什么“P”被排版在原点？

\documentclass[10pt]{amsart}

\usepackage{mathtools,array}

\usepackage{tikz}
\usetikzlibrary{calc}





\begin{document}



\begin{center}
\begin{tikzpicture}



%Part of an ellipse and a parabola are drawn.
\draw (0,0) arc (0:90: 2 and 1);
\draw (0,0) arc (0:-72: 2 and 1);
\path node[anchor=east] at ({-2+sqrt(31)/8},-15/16){$E_{1}$};
\draw[fill] (0,0) circle (1.5pt);
%A "pin" is drawn between (0,0) and its label.
\draw[draw=gray, line width=0.8pt, shorten <=1.5mm, shorten >=0mm] (0,0) -- ({(atan(1/3)+90)/2}:0.75);
\path node[anchor=south] ({(atan(1/3)+90)/2}:0.75){$P$};
%
\draw[domain=-1:4, smooth, variable=\x, blue] plot ({\x}, {-1/9*\x*\x + 1/3*\x});
\path node[anchor=south west, xshift=-0.5mm, yshift=-1mm] at (3.75,-5/16){$E_{2}$};

\end{tikzpicture}
\end{center}


\end{document}

Answer 1

你遗漏了at：

\路径节点[锚点=南]在({(atan(1/3)+90)/2}:0.75) {$P$};

您刚刚命名了该节点{(atan(1/3)+90)/2}:0.75。

不过，由于您已经有了到该坐标的路径，因此您只需在那里添加节点即可：

\draw[draw=gray, line width=0.8pt, shorten <=1.5mm, shorten >=0mm] (0,0)
  -- ({(atan(1/3)+90)/2}:0.75) node[anchor=south] {$P$};

---

我擅自用arcPGFkeys 支持的较新语法重写了您的图片，该语法允许您设置特定值，例如半径，外部arc[]它将用于路径/范围/组中包含的所有圆弧。

我也用过parabola路径操作它不仅可以让您指定弯曲，还可以指定其位置或抛物线的高度（有关更多信息，请参阅手册）。

TikZ 还具有pin选项与坐标和标签一起使用，但它可能不够精确，这取决于角度的来源(atan(1/3)+90)/2。

代码

\documentclass{standalone}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}
\draw ([shift=(left:2)] -72:2 and 1) node[left]{$E_1$}
      arc [start angle=-72, end angle=90, x radius=2, y radius=1];

\draw[fill] (0,0) circle [radius=1.5pt];

\draw[draw=gray, line width=0.8pt, shorten <=1.5mm, shorten >=0mm] (0,0)
  -- ({(atan(1/3)+90)/2}:0.75) node[anchor=south] {$P$};

\draw[blue] (-1, -1/9-1/3) parabola bend (1.5, -.25+.5) (4, -16/9+4/3) node[right] {$E_2$};
\end{tikzpicture}
\end{document}

Answer 2

\documentclass[tikz, border=1cm]{standalone}
\begin{document}
\begin{tikzpicture}
\draw (0,0) arc[start angle=0, end angle=90, x radius=2, y radius=1];
\draw (0,0) arc[start angle=0, end angle=-72, x radius=2, y radius=1] node[left]{$E_{1}$};
\draw[blue] plot[domain=-1:4, smooth] ({\x}, {-1/9*\x*\x + 1/3*\x}) node[above]{$E_{2}$};
\fill (0,0) circle (1.5pt);
\draw[draw=gray, thick, shorten <=1.5mm] (0,0) -- (50:0.8) node[above right, inner sep=1pt]{$P$};
\end{tikzpicture}
\end{document}