[tex]\text{Iti arat cum se rezolva prima(restul sunt asemanatoare):}\\a)\text{Notam suma cu S}:\\S=3+3^2+3^3+\ldots+3^{48} \\\text{Inmultind aceasta suma cu S se obtine:}\\3S=3^2+3^3+3^4+\ldots+3^{49}\\\text{Scadem S din 3S:}\\3S-S=3^{49}-3\\2S=3^{49}-3\\\boxed{S=\dfrac{3^{49}-3}{2}}[/tex]
