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Aflați termenul de rangul 20 al progresiei aritmetice:a6=-21,a50=-197

Răspuns :

a₆=a₁+5r=-21           a₁=-21-5r
a₅₀=a₁+49r=-197      a₁=-197-49r
                            -21-5r=-197-49r
                             49r-5r=-197+21
                             44r=-176
                              r=-4    ⇒a₁=-21-5×(-4)=-21+20=-1

a₂₀=a₁+19r=-1+19×(-4)=-1-76=-77