-1.5 6.5 setxrange
-1.5 4.5 setyrange

/a 3 def

/A {0 a} def
/B {a a} def
/C {a 0} def
/D {0 0} def

/I {C D 60 rotatepoint} def
/J {C B 60 rotatepoint} def

[A B C D I J] points
[A B C D] polygone
[D I C J B] ligne

[
   A B
   B C
   C D
   D A
   D I
   I C
   B J
   J C
] {1 marked} dapply


bleu
D A B angledroit

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