\usemodule[tikz]         
\usemodule[tikz-cd]

\starttext

\starttikzpicture[      scale=3,line cap=round
                        axes/.style=,         
                        important line/.style={very thick},
                        information text/.style={rounded corners,fill=red!10,inner sep=1ex} ]

        \draw[help lines,step=0.5cm] (-1.4,-1.4) grid (1.4,1.4);
        \draw (0,0) circle (1cm);                               
                \draw[->] (-1.5,0) -- (1.5,0) node[right] {$x$} coordinate(x axis);
                \draw[->] (0,-1.5) -- (0,1.5) node[above] {$y$} coordinate(y axis);
                \foreach \x/\xtext in {-1, -.5/-\frac{1}{2}, 1}                    
                        \draw[xshift=\x cm] (0pt,1pt) -- (0pt,-1pt)                
                                node[below,fill=white] {$\xtext$};
                \foreach \y/\ytext in {-1, -.5/-\frac{1}{2}, .5/\frac{1}{2}, 1}
                        \draw[yshift=\y cm] (1pt,0pt) -- (-1pt,0pt)
                                node[left,fill=white] {$\ytext$};
        \filldraw[fill=green!20,draw=green!50!black] (0,0) -- (3mm,0pt) arc(0:30:3mm);
        \draw (15:2mm) node[green!50!black] {$\alpha$};
        \draw[important line,red]
                (30:1cm) -- node[left=1pt,fill=white] {$\sin \alpha$} (30:1cm |- x axis);
        \draw[important line,blue]
                (30:1cm |- x axis) -- node[below=2pt,fill=white] {$\cos \alpha$} (0,0);
        \draw[important line,orange!80!black] (1,0) -- node[right=1pt,fill=white] {
                $\displaystyle \tan \alpha \black =
                \frac{{\red \sin \alpha}}{\blue \cos \alpha}$}
                (intersection of 0,0--30:1cm and 1,0--1,1) coordinate (t);
        \draw (0,0) -- (t);
        \draw[xshift=1.85cm]
                node[right,text width=6cm,information text]
                {
                        The {\darkgreen angle $\alpha$} is $30^\circ$ in the
                        example ($\pi/6$ in radians). The {\red sine of
                        $\alpha$}, which is the height of the red line, is
                        \startformula
                        {\red \sin \alpha} = 1/2.
                        \stopformula
                        By the Theorem of Pythagoras ...
                };
\stoptikzpicture

\stoptext