tkz-euclide support

[MatrixManAtYrService]

Use like this:

%%tikz -eu

\coordinate (O) at (0,0); 
\coordinate (A) at (4,0); 
\coordinate (B) at (0,2); 
\draw (O)--(A)--(B)--cycle;

\node (a) at (1.4,2.1) {$90^{\circ}-\theta$}; 
\node (b) at (0.1,1.8){}; 
\path (a) edge [->,bend left=45,looseness=0.8] (b); 

\tkzLabelSegment[below=2pt](O,A){$b$}
\tkzLabelSegment[left=2pt](O,B){$a$}
\tkzLabelSegment[above right=2pt](A,B){$c$}
\tkzMarkRightAngle[size=0.3](A,O,B)

\tkzLabelAngle[pos = 1.0](O,A,B){$\theta$}

(source: https://tex.stackexchange.com/q/499338/258552)

[MatrixManAtYrService]