Automatically draw and labels angles of a triangle in TikZ

Question

I'm using LaTeX with TikZ to create a (right-angle) triangle. Now I would like to label each angle by adding a half circle and a text label.

Unfortunately I only managed to create a label for the gamma angle. What do I need to do in order to put such a label into the remaining corners?

\begin{tikzpicture}[thick]
\draw(0,0) -- (90:2cm) node[midway,left]{$opposite leg$} -- (0:4cm) node[midway,above right]{$hypotenuse$} -- (0,0) node[midway,below]{$adjacent leg$};
\draw[fill=lightgray, thick] (0,0) -- (0:0.8cm) arc (0:90:0.8cm) node at (45:0.5cm) {$\gamma$} -- cycle;
\end{tikzpicture}

Welcome to TeX.sx! No need to add thanks, simply upvote good replies to your questions — Peter Jansson, Feb 02 '13 at 13:30

score 32 · Accepted Answer · edited Jun 10 '20 at 12:32

I'd use tkz-euclide for this task. It provides a nice macro \tkzMarkAngle which is really of help in this case.

The code:

\documentclass[tikz,border=2pt,png]{standalone}
\usepackage{tkz-euclide}
\usetkzobj{all}
\begin{document}
\begin{tikzpicture}[thick]
\coordinate (O) at (0,0);
\coordinate (A) at (4,0);
\coordinate (B) at (0,2);
\draw (O)--(A)--(B)--cycle;
\tkzLabelSegmentbelow=2pt{\textit{adjacent leg}}
\tkzLabelSegmentleft=2pt{\textit{opposite leg}}
\tkzLabelSegmentabove right=2pt{\textit{hypotenuse}}
\tkzMarkAnglefill= orange,size=0.65cm,%
opacity=.4
\tkzLabelAnglepos = 0.35{$\gamma$}
\tkzMarkAnglefill= orange,size=0.8cm,%
opacity=.4
\tkzLabelAnglepos = 0.6{$\alpha$}
\tkzMarkAnglefill= orange,size=0.7cm,%
opacity=.4
\tkzLabelAnglepos = 0.5{$\beta$}
\end{tikzpicture}
\end{document}

The result:

enter image description here

Disclaimer

Hoping that the labels are right.

As Torbjørn T. was suggesting in its comment, it is even possible to create square angles thanks to the macro \tkzMarkRightAngle. The previous example becomes:

\documentclass[tikz,border=2pt,png]{standalone}
\usepackage{tkz-euclide}
\usetkzobj{all}
\begin{document}
\begin{tikzpicture}[thick]
\coordinate (O) at (0,0);
\coordinate (A) at (4,0);
\coordinate (B) at (0,2);
\draw (O)--(A)--(B)--cycle;
\tkzLabelSegmentbelow=2pt{\textit{adjacent leg}}
\tkzLabelSegmentleft=2pt{\textit{opposite leg}}
\tkzLabelSegmentabove right=2pt{\textit{hypotenuse}}
\tkzMarkRightAnglefill=orange,size=0.5,opacity=.4% square angle here
\tkzLabelAnglepos = 0.35{$\gamma$}
\tkzMarkAnglefill= orange,size=0.8cm,%
opacity=.4
\tkzLabelAnglepos = 0.6{$\alpha$}
\tkzMarkAnglefill= orange,size=0.7cm,%
opacity=.4
\tkzLabelAnglepos = 0.5{$\beta$}
\end{tikzpicture}
\end{document}

The result:

enter image description here

That's an awesome solution. (And yes, the labels are right. ;-)) — Patrick, Feb 02 '13 at 13:57
@T.Verron: that's the first thing done after having drawn the triangle :) — Claudio Fiandrino, Feb 03 '13 at 08:53
I know the OP specified an arc also for the right angle \gamma, but I'll just mention that if one wants a square instead, then one can use \tkzMarkRightAngle[fill=orange,size=0.5,opacity=.4](A,O,B). — Torbjørn T., Aug 24 '13 at 18:19
@TorbjørnT.: it might be a nice addition, thanks. I'll edit as soon as possible. — Claudio Fiandrino, Aug 25 '13 at 10:20

score 11 · Answer 2 · answered Feb 02 '13 at 15:00

A solution with 'pure' tikz could be:

\documentclass[tikz]{standalone}

\usetikzlibrary{calc}

\begin{document}
\begin{tikzpicture}[thick]
\draw(0,0) -- (90:2cm) node[midway,left]{$opposite\ leg$} -- (0:4cm) node[midway,above right]{$hypotenuse$} -- (0,0) node[midway,below]{$adjacent\ leg$};
\draw[fill=lightgray, thick] (0,0) -- (0:0.8cm) arc (0:90:0.8cm) node at (45:0.5cm) {$\gamma$} -- cycle;
\draw[fill=lightgray, thick] (4,0) -- ++(180:0.8cm) arc (180:180-atan2(4,2):0.8cm) node at ($(167:0.6cm)+(4,0)$) {$\alpha$} -- cycle;
\draw[fill=lightgray, thick] (0,2) -- ++(-90:0.8cm) arc (-90:-90+atan2(2,4):0.8cm) node at ($(-60:0.5cm)+(0,2)$) {$\beta$} -- cycle;
\end{tikzpicture}
\end{document}

Note that you need the calc library and you need to tweak the coordinates by hand if you change the size of the triangle. Of course you can parametrize to some extent by defining lengths (e.g. \def\adjleg{4} and \def\oppleg{2}) and use these in the figure. The angles are a bit more difficult, though.

enter image description here

This does not seem to work for me. I get this image instead using TeXLive2013. — Peter Grill, Jun 08 '14 at 05:47
There was a change in the atan2 function from v2.x to v3 of pgf. The order of the arguments was swapped from atan2(x,y) to (the more common) atan2(y,x). So the first should now be atan2(2,4), and the second atan2(4,2), not the other way around. — Torbjørn T., May 07 '18 at 10:51

Automatically draw and labels angles of a triangle in TikZ

2 Answers2

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