Răspuns:
Explicație pas cu pas:
S=1+3+3²+3³+......+3⁷⁸+3⁷⁹+3⁸⁰=(1+3+3²)+3³(1+3+3²)+.....+3⁷⁸(1+3+3²)= (1+3+3²)ₓ(1+3³+.....+3⁷⁸)=13ₓ(1+3³+....+3⁷⁸)
S:13=1+3+...+3⁷⁸ rest 0
3S=3+3²+3³+3⁴+.....+3⁸⁰+3⁸¹ dar S-1=3+3²+3³+.....+3⁸⁰
3S=S-1+3⁸¹
2S=3⁸¹-1
2S=(3³)²⁷-1
2S=27²⁷-1>27²⁷-9
