Câu hỏi 12Tính giá trị $$\sum_{j=2}^{\infty}\left(\sum_{i=1}^{\infty}\frac1{9^i}\right)^{j/3}.$$$1$$\sqrt[3]{\frac17}$$\frac12\times\sqrt[3]{\frac17}$$\frac12$$\frac17$