The following sum of numbers (expressed in decimal notation)
$$1+11+111+\cdots +\underset{n}{\underbrace{11\dots1}}$$
is equal to
- $\left ( 10^{n+1}-10-9n \right )/81$
- $\left ( 10^{n+1}-10+9n \right )/81$
- $\left ( 10^{n+1}-10-n \right )/81$
- $\left ( 10^{n+1}-10+n \right )/81$
