20 setxunit
-5.5 5.5 setxrange
-4 1 setyrange

/quadrillagegray .2 def
quadrillage

/A {-4 -2} def
/B {0 0} def
/C {4 -2} def

A
B C vecteur 
.5 mulv 
addv
/N defpoint

1.3 setlinewidth
[A B C] polygone
N times2

setTimesItalic
   (A) A dltext
   (B) B ultext
   (C) C drtext
   (N) N drtext