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Aratati ca nr. a= (3^21+3^20+3^19) este patrat perfect. va roooog repede dau coroana!!

Răspuns :

a= (3^21+3^20+3^19)/39=3^18(3^3+3^2+3^1)/39=3^18(27+9+3)/39=
=
3^18 ×39/39=3^18=(3^9)^2    deci patrat perfect