Label angle with tikz

Question

I am able to draw a nice right triangle with

\begin{tikzpicture}[thick]
  \draw (0,0)  -- (4,0) node[midway,below]{$f$} -- (4,2) node[midway,right]{$\frac{x}{2}$}-- cycle;
\end{tikzpicture}

Now I want to label the left angle of the triangle and I am stuck on how to do this.

Answer 1

My answer shows how to label the vertex and the angle. The macro tikzAngleOfLineis not optimized

\documentclass{minimal}
\usepackage{tikz}
\usetikzlibrary{calc}

\begin{document}

\newcommand{\tikzAngleOfLine}{\tikz@AngleOfLine}
  \def\tikz@AngleOfLine(#1)(#2)#3{%
  \pgfmathanglebetweenpoints{%
    \pgfpointanchor{#1}{center}}{%
    \pgfpointanchor{#2}{center}}
  \pgfmathsetmacro{#3}{\pgfmathresult}%
  }

  \begin{tikzpicture}
    \coordinate (A) at (1,1);
    \coordinate (B) at ($(A)+(25:3)$);
    \coordinate (C) at ($(A)+(100:5)$);
    \draw (A) node[left]{$A$} -- (B) node[right]{$B$}node[midway,below]{$c$} -- (C)node[above]{$C$}node[midway,above]{$a$} -- (A)node[midway,left]{$b$};

    \tikzAngleOfLine(A)(B){\AngleStart}
    \tikzAngleOfLine(A)(C){\AngleEnd}
    \draw[red,<->] (A)+(\AngleStart:2cm) arc (\AngleStart:\AngleEnd:2 cm);
    \node[circle,fill=green] at ($(A)+({(\AngleStart+\AngleEnd)/2}:1 cm)$) {$\alpha$};
\end{tikzpicture}
\end{document} 

Answer 2

I have searched for many solutions. Finally, no one is satisfactory.

Solution 1: as shown in here, use

\tkzMarkAngle and \tkzLabelAngle

Problem: can't draw an arrow on the arc.

Solution 2: as show in the same place, use

\begin{scope}
  \path[clip] (A) -- (E) -- (F) -- cycle;
  \draw [red, fill=red!20] (A) circle (10pt);
\end{scope}

Problem: can't draw an arrow on the arc.

Solution 3: the solution in the accepted answer might be complicated for a beginner. And I tried the code, it seems the direction of the arc is not what I expected.

==================================================

My solution: Of course there are many other good solutions. Regarding the problem I am interested in, I came up with a simple solution. I believe many people already know about this. Just post it here for a memo.

%Requires \usetikzlibrary{calc} in the preamble
\begin{tikzpicture}
    \coordinate (B) at (2.345,-1.234);
    \coordinate (A) at (0,0);
    \coordinate (C) at (-2.1,3.2);
    \draw [densely dotted, thick] (B)--(A)--(C);
    \draw (A) node[below] {$A$};
    \draw (B) node[below] {$B$};
    \draw (C) node[below left] {$C$};
    % label the angle subtended at p1 by p2 and p3
    \def\angleRadius{30pt}
    % draw the arc
    \draw[red,->,thick] let \p1=(A), \p2=(B), \p3=(C), 
        \n1={atan2(\x2-\x1,\y2-\y1)}, \n2={atan2(\x3-\x1,\y3-\y1)} in
        ($(\p1)!\angleRadius!(\p2)$) arc (\n1:\n2:\angleRadius);
    % draw the label
    \draw[red] let \p1=(A), \p2=(B), \p3=(C), 
         \n1={atan2(\x2-\x1,\y2-\y1)}, \n2={atan2(\x3-\x1,\y3-\y1)} in
         (\p1)+(\n1/2+\n2/2:\angleRadius) node[above] {$\alpha$};
\end{tikzpicture}

Result:

We can use the following code to label the angle which is larger than 180.

\def\angleRadius{30pt}
\draw[red,->,thick] let \p1=(A), \p2=(B), \p3=(C), \n1={atan2(\x2-\x1,\y2-\y1)}, \n2={atan2(\x3-\x1,\y3-\y1)} in
    ($(\p1)!\angleRadius!(\p2)$) arc (\n1:\n2-360:\angleRadius);
\draw[red] let \p1=(A), \p2=(B), \p3=(C), \n1={atan2(\x2-\x1,\y2-\y1)}, \n2={atan2(\x3-\x1,\y3-\y1)} in
    (\p1)+(\n1/2+\n2/2+180:\angleRadius) node[below] {$\alpha$};

The result is:

Features of the solution:

1. Simple. Simply input the three points that determine the angle.
2. the arc is generated by arc command. Can set the arrow of the arc and of course other properties.
3. Require \usetikzlibrary{calc}, but no \usepackage{tkz-euclide}.

Answer 3

You can insert a node at the start of the triangle, at (0,0), and put a label to the left of it with node[left]{$A$}. A full example:

\documentclass{article}

\usepackage{tikz}

\begin{document}

\begin{tikzpicture}[thick]
  \draw (0,0) node[left]{$A$}  -- (4,0) node[midway,below]{$f$} -- (4,2) node[midway,right]{$\frac{x}{2}$}-- cycle;
\end{tikzpicture}

\end{document}