这是等比数列啊S(n) = Σ a^n = a^1 + a^2 + a^3 + ... + a^n公比r = a^2/a^1 = a^3/a^2 = a所以通项an = ar^(n - 1) S(n) = a + ar + ar^2 + ar^3 + ... + a^(n - 2) + a^(n - 1)r * S(n) = ar + ar^2 + ...