ClearAll[l,f];

tplet={3,4,5};
DeltaPyth={{tplet[[1]],0},{tplet[[1]],tplet[[2]]},{0,tplet[[1]]},{-tplet[[2]],tplet[[1]]}};
TailleMaille={{l,0},{0,l}};
Equations=Table[FullSimplify[f*(-Sin[theta]*DeltaPyth[[k,1]]+Cos[theta]*DeltaPyth[[k,2]])*(TailleMaille[[j,1]]*Cos[theta]+TailleMaille[[j,2]]*Sin[theta]+(Cos[theta]*DeltaPyth[[k,1]]+Sin[theta]*DeltaPyth[[k,2]])/2)],{k,1,4},{j,1,2}];

TestF=Do[rule={f->j/100,theta->Pi/8.,l->23};eq=Equations/.rule;
If[Union[Map[EqualZero,eq]]=={True},Print["f = ",j/100]],{j,0,100}]

EqualZero[x_]:=(x[[1]]==0)&&(x[[2]]==0);

eqq=Simplify[Equations/.l->23/.theta->3*Pi/4]

eqq=FullSimplify[Equations]

FullSimplify[ Sin[theta] * Cos[theta]]