Problème Latex

Bonjour,

Je ne comprends pas mon erreur, car la ligne 72 ou il y a trois erreurs est vide.

Message d'erreur :

! Missing $ inserted.
<inserted text>
$
l.72
I've inserted a begin-math/end-math symbol since I think
you left one out. Proceed, with fingers crossed.
! Missing } inserted.
<inserted text>
}
l.72
I've inserted something that you may have forgotten.
(See the <inserted text> above.)
With luck, this will get me unwedged. But if you
really didn't forget anything, try typing '2' now; then
my insertion and my current dilemma will both disappear.
Underfull \hbox (badness 10000) in paragraph at lines 66--72
[]
! Extra }, or forgotten \endgroup.
\par ...m \@noitemerr {\@@par }\fi \else {\@@par }
\fi
l.72
I've deleted a group-closing symbol because it seems to be
spurious, as in '$x}$'. But perhaps the } is legitimate and
you forgot something else, as in '\hbox{$x}'. In such cases
the way to recover is to insert both the forgotten and the
deleted material, e.g., by typing 'I$}'.

Savez-vous pourquoi ils affichent ce message? Merci