Câu 46. Tính tổng \[ 1 + \frac{1}{2} + \frac{1}{4} + \frac{1}{2^n} + \ldots \] a, \[ b, S = 2 + \frac{2}{3} + \frac{2}{3^2} + \ldots + \frac{2}{3^n} + \ldots \]