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3^x + 3^x-1 + 3^x-2 + 3^x-3=40
x=?


Răspuns :

3^(x-3) * (3³+3²+3^1+1)=40
 3^(x-3) * (27+9+3+1)=40
 3^(x-3)*40=40
 3^(x-3)=1
x-3=0
x=3
as simple as that!