autocrop
-2 5 setxrange
-2 5 setyrange

/A {0 0} def
/C {4.2 0} def
A 2.8 C 3.9 intercercle /B defpoint popp
/I {A B milieu} def

%% l'angle ACB
180 C B angle sub /alpha exch def

%% la droite (ID)
I dupp alpha dir 
A C interdroite
/D defpoint

[I D] ligne
A I D .5 (-) tripointarcarrow
B C A .5 (-) tripointarcarrow

A I 1 marked
I B 1 marked
1.5 setlinewidth
[A B C] polygone

setTimesItalic
   (A) A dltext
   (B) B urtext
   (C) C drtext
   (D) D drtext
   (I) I ultext