%% auteur : gh
100 setxunit
-1 2 setxrange
-1 3 setyrange

/cosa {pi 3 div Cos} def
/sina {pi 3 div Sin} def
/O {0 0} def
/A {cosa sina} def
/B {0 sina sub cosa} def
/M {2 cosa mul 2 sina mul} def
/Mx {2 cosa mul 0} def
/My {0 2 sina mul} def

tracerepere
16 setfontsize
setTimesItalic
   (O) O dltext

<tex> 
$$
\vec{\imath }
$$ 
</tex> 
0.5 0  [1.5 1.5] dctexlabel 

<tex> 
$$
\vec{\jmath }
$$ 
</tex> 
0 0.5  [1.5 1.5] cltexlabel 

bleu
1 setlinewidth
continu
O M droite

<tex> 
$$
D_\alpha
$$ 
</tex> 
0.9 1.8  [1.5 1.5] {60} cctexlabel 

setSymbol
   (a) 0.45 0.25  dctext
/arrowangle -5 store
1 0 O A 0.4 (->) tripointarc