Tổng S = 11 + 11 + 111 + ... + 11...111 là S = 10/81 (10^n-1 -1) -n/9

A. $S=\frac{10}{81}\left({10}^{n-1}-1\right)-\frac{n}{9}.$

B. $S=\frac{10}{81}\left({10}^n-1\right)+\frac{n}{9}.$

C. $S=\frac{1}{81}\left({10}^n-1\right)-\frac{n}{9}.$

D. $S=\frac{10}{81}\left({10}^n-1\right)-\frac{n}{9}.$