autocrop
12 setxunit

/A {-4 -3} def
/B {4 -3} def
/C {-1 4} def

/P {-2 -3} def
/R {-5 A C xdpoint} def
/Q {0 C B ydpoint} def
[-2 0 2] {-3 point} apply
[-3 -2] {A C xdpoint point} apply
[-2 3 1 stepto] {C B ydpoint point} apply

A B Q R interdroite /P defpoint

[A B C R] points

[Q R A] (-) ligne
[A B C] polygone

setTimesItalic
  (A) A cltext
  (B) B crtext
  (C) C uctext
  (P) P drtext
  (R) R cltext
  (Q) Q urtext