I can draw the right angle.
Code: Select all
\begin{document}
\begin{tikzpicture}[dot/.style={fill,circle,inner sep=1.5pt}]
\coordinate (Q) at (0,0);
\coordinate (A) at ($(Q)+(0:4)$);
\coordinate (P) at ($(Q)+(90:3)$);
\draw (A) -- (Q) -- (P) -- cycle;
\path (A) -- node [midway,sloped,above] {r} (P);
\path (A) -- node [midway,sloped,below] {a} (Q);
\path (P) -- node [midway,sloped,below] {x} (Q);
***\angle {A} -- ["$\theta$",draw,,angle radius=1cm] {angle = Q--A--P}***
\tkzMarkRightAngle(A,Q,P);
\end{tikzpicture}
\end{document}