-2.5 4.5 setxrange
-0.5 3 setyrange
50 setxunit
50 setyunit

/A {0 0} def
/B {4 0} def
/C {-1 2} def
/D {-1 0} def


C D B angledroit
[A B C] polygone

gsave
pointilles
[C D] ligne
[A D] ligne
grestore

<latex>
$a$
</latex>
2.1 0.9 [1.8 dup] ultexlabel

<latex>
$b$
</latex>
-0.75 0.8 [1.8 dup] crtexlabel

<latex>
$c$
</latex>
2 0.05 [1.8 dup] dctexlabel

(A) A [1.5 dup] dltext
(B) B [1.5 dup] drtext
(C) C [1.5 dup] uctext
(C') D [1.5 dup] dctext


<latex>
$b)$
</latex>
-1.75 0.8 [2.0 dup] ultexlabel