k
( b n + b n − 1 − 1 ) ( b k − 1 )
5
109999 × 99999 = 10999790001
6
109999 × 999999 = 109998890001
7
109999 × 9999999 = 1099989890001
8
109999 × 99999999 = 10999899890001
9
109999 × 999999999 = 109998999890001
10
109999 × 9999999999 = 1099989999890001
11
109999 × 99999999999 = 10999899999890001
12
109999 × 999999999999 = 109998999999890001