Notezi S=1+2+2^2+....+2^2018
2S=2+2^2+....+2^2018+2^2019
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2S-S=2+2^2+,,,,+2^2018+2^2019-1-2-,,,-2^2018=2^2019-1
n=1+2^2019-1=2^2019==2^(2016+3)
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Determini ultima cifra a lui 2^k
2^1=2
2^2=4
2^3=8
2^4=16 u(16)=6
2^5=32 U(32)=2
2^6=64 U(64)=4
2^5=128 U(128=8
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Observi ca ultima cifra se repeta din 4 in 4
Un numar de forma 2^(4k+3) are ultima cifra 8 unde 4k=2016
Deci U(n)=8
