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