-0.5 5.5 setxrange
-0.5 3 setyrange
50 setxunit
50 setyunit

/A {1.1 2.5} def
/B {0 0} def
/C {5 0} def
/M {2.2 0.75} def

/P {M B C orthoproj} def
/Q {M A C orthoproj} def
/R {M A B orthoproj} def

[A B C] polygone

M P C angledroit
M Q A angledroit
M R B angledroit

(A) A [1.5 dup] uctext
(B) B [1.5 dup] dltext
(C) C [1.5 dup] drtext
(M) 2.2 0.8 [1.5 dup] dltext

[M P] ligne
[M Q] ligne
[M R] ligne

%gsave
%pointilles
%[M A] ligne
%[M B] ligne
%[M C] ligne
%grestore

<latex>
$a$
</latex>
2.5 0 [1.8 dup] dctexlabel

<latex>
$b$
</latex>
3.15 1.7 [1.8 dup] dctexlabel

<latex>
$c$
</latex>
0.3 1.4 [1.8 dup] dctexlabel

<latex>
$d_a$
</latex>
2.18 0.2 [1.4 dup] urtexlabel

<latex>
$d_b$
</latex>
2.5 1.18 [1.4 dup] cltexlabel

<latex>
$d_c$
</latex>
1.4 1.1 [1.4 dup] dctexlabel