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Aratati ca numarul a=(3^21+3^20+3^19):39 este patrat perfect....

Răspuns :

a=(3^21+3^20+3^19):39=
=3^19×(3^2+3^1+3^0):(3×13)=
=3^18×(9+3+1):13=
=3^18×13:13=
=3^18=
=
(3^9)²  deci patrat perfect