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a=2Sn/3 (1)
Sn are n termeni
Sn=1+1/3+1/3^2+1/3^3+......................+1/3^(n-1)
Sn/3=1/3 + 1/3^2+1/3^3+1/3^4+............+1/3^n
Sn-Sn/3= 2Sn/3 = 1-1/3^n = (3^n - 1)/3^n (vezi reducerea termenilor)
revenim la relatia (1)
a=2Sn/3 =(3^n-1)/3^n, varianta a.