The following is a sequence of formula,
$$\begin{align*} \begin{matrix} & 9*1+2 &= &11 \\ & 9*12+3 &= &111 \\ & 9*123+4 &= &1111 \\ & 9*1234+5 &= &11111 \\ \end{matrix} \\ \dots \dots \dots \dots \dots \dots \\ \end{align*}$$
Here numbers are in base $10$.
(a) Establish a formula in $\sum$ notation.
(b) Generalize that formula in for any base $b$..