TikZ: Create right angle triangle with angle labels

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}

Answer 1

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;

\tkzLabelSegment[below=2pt](O,A){\textit{adjacent leg}}
\tkzLabelSegment[left=2pt](O,B){\textit{opposite leg}}
\tkzLabelSegment[above right=2pt](A,B){\textit{hypotenuse}}

\tkzMarkAngle[fill= orange,size=0.65cm,%
opacity=.4](A,O,B)
\tkzLabelAngle[pos = 0.35](A,O,B){$\gamma$}

\tkzMarkAngle[fill= orange,size=0.8cm,%
opacity=.4](B,A,O)
\tkzLabelAngle[pos = 0.6](B,A,O){$\alpha$}

\tkzMarkAngle[fill= orange,size=0.7cm,%
opacity=.4](O,B,A)
\tkzLabelAngle[pos = 0.5](O,B,A){$\beta$}


\end{tikzpicture}
\end{document}

The result:

Disclaimer

Hoping that the labels are right.

Answer 2

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.