S = 1 - [ 1 /3 - 1/3² + 1 /3³ + ....... ]
↓ suma de n termeni cu b₁ = 1 /3 in prog. geom
q = -1 /3
suma pg = 1 /3 · [ ( -1 /3 ) la n - 1 ] / [ ( -1 /3 ) -1 ]
= 1 /3 · [ ( -1/3 ) la n -1 ] / [ - 4 /3 ]
= - [ ( -1/3) la n - 1 ] / 4
S = 1 - { - [ ( -1 /3) la n - 1 ] / 4 } = 1 + [ ( - 1 /3) la n - 1 ] / 4
daca n --> ∞ ↓
= 0
S = 1 - 1 /4 = 3 /4
[S] = 0
