-10 8 setxrange
-5 7 setyrange

/I {2 -1} def
/alpha 50 def
/v {6 5} def

/A {0 0} def
/B {0 4.5} def
/C {-9 3 sqrt mul 2 div 0} def

/A1 {A I alpha rotatepoint} def
/B1 {B I alpha rotatepoint} def
/C1 {C I alpha rotatepoint} def

/P {A1 v addv} def
/M {B1 v addv} def
/N {C1 v addv} def

[A B C] polygone
C A B angledroit

[P M N] polygone
N P M angledroit

%% [A1 B1 C1] points
%% [P M N] (times2) plot

A C B 1.5 (-) tripointarc
N M P 1 (-) tripointarc

setTimesItalic
   (A) A drtext
   (B) B urtext
   (C) C dltext
   (P) P rtext
   (M) M utext
   (N) N dtext
setTimes
   (30°) C 1.65 .35 addv text
   (60°) M 0 -1.5 addv text
   (4,5) M P milieu utext

<tex>
$$
   {9\sqrt 3\over 2}
$$
</tex>

A C milieu dctexlabel