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2 x (1+3+3 la 2 + ... + 3 la 49)

Răspuns :

2 (1+3^1+3^2+.....+3^49)=2*(3^50-1)/2=3^50-1
2*(1+3+3^2+.....+3^49)=
=2*(3^50-1):2=3^50-1