na9der n3awnek fel moutataliyat lhissabiya ;kayen 3 9awanin tekhdem bihem w taba9hem 3la koul temrin :
al bourhan:Un+1-Un=R(r=al assas)
al had l3am:idha lhad lawel ta3 lmoutataliya Uo ndirou:Un=Uo+nr
idha lhad lawel U1 ndirou:Un=U1+(n-1)r
al majmou3:kif kif 3la hssab lhad lawel
Sn=n+1/2(Uo+Un)
Sn=n/2(U1+Un)