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Sara68
a fost răspuns

Fie S= 1+3+3^2+3^3+...+3^80

demonstrati ca 2S>27^27-9

Răspuns :

Incognito
 S=1+3+3^2+...+3^80
3S=3+3^2+3^3+...+3^81
......................................... (-)
2S=3^81-1=(3^3)^27-1=27^27-1>27^27-9