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

/cosa {pi 6 div Cos} def
/sina {pi 6 div Sin} def
/F {0 0} def
/M {cosa sina} def
/Mx {cosa 0} def
/Hx {2 0} def
/H {2 sina} def

traceaxes
axesarrow

1.7 setcurvelinewidth
continu
bleu

H Hx droite
16 setfontsize
setTimesItalic
   (F) F dltext
   (D) Hx 0 -1 addv ultext

1.2 setcurvelinewidth
rouge
[M H] points
setTimesItalic
   (M) M ultext
   (H) H crtext
F M (-) line
pointilles
M H (-) line
0.7 setcurvelinewidth
noir
M Mx (-) line

continu
/arrowangle -5 store
1 0 F M 0.4 (->) tripointarc
setSymbol
(q) F pi 12 div dup Cos exch Sin 0.4 mulv addv crtext

<tex> 
$$
d-\rho\cos\theta
$$ 
</tex> 
M H milieu  [1.5 1.5] uctexlabel 

<tex> 
$$
d
$$ 
</tex> 
F Hx milieu  [1.5 1.5] dctexlabel 

<tex> 
$$
\rho
$$ 
</tex> 
F M milieu  [1.5 1.5] {30} uctexlabel