I can't display math images

[anhduc109dk]

I'm trying to display geometric shapes in mathematics but I get an error environment: tikzpicture

this is the tex i need to display, can you assist me,
\begin{tikzpicture}\small
\def\r{1.8}
\coordinate[label=$A$] (A) at (0.5*\r,0.8*\r);
\coordinate[label=below:$B$] (B) at (-\r,0);
\coordinate[label=below:$C$] (C) at (\r,0);
\draw[thin] (A) -- node[above] {$c$}
node[pos=0.03,below,inner sep=4] {$\alpha$}
(B) -- (C) -- node[right] {$b$} (A);
\end{tikzpicture}
Thank you