Tổng \(S = 1 + 11 + 111 + ... + \underbrace {11......

Câu hỏi: Tổng \(S = 1 + 11 + 111 + ... + \underbrace {11...111}_{n\,số\,1}\)  là :

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