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determinati ultima cifra a numarului:
x=2⁰+2¹+2²+2³+...+2¹⁹⁹⁹+2²⁰⁰⁰


Răspuns :

[tex]x=2^0+2+2^2+2^3+....+2^{1999}+2^{2000}\\ x=(2^0+2+2^2+2^3)+2^4 ( 1+2+2^2+2^3)+....+2^{1997}(1+2+2^2\\ +2^3)\\ x=(1+2+2^2+2^3)(1+2^4+..+2^{1997})\\ x=15(1+2^4+...+2^{1997})\\ Ultima\ cifra\ este\ 5.[/tex]