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1+5+5^1+5^2+5^3+5^4+...+5^2018???

Răspuns :

S = 1+5+5^1+5^2+5^3+5^4+...+5^2018

S = 5^0+5^1+5^2+5^3 +...+ 5^2017+5^2018 |×5

5 × S = 5^1+5^2+5^3+5^4 +...+ 5^2018 + 5^2019

5 × S - S = 5^2019 - 5^0

4 × S = 5^2019 - 1

S = ( 5^2019 - 1) / 4
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