k

$\left(b^{2n-1}+b^n-1\right)\left(b^k-1\right)$

5

$1000099999\times 99999=100008999800001$

6

$1000099999\times 999999=1000098998900001$

7

$1000099999\times 9999999=10000998989900001$

8

$1000099999\times 99999999=100009998899900001$

9

$1000099999\times 999999999=1000099997999900001$

10

$1000099999\times 9999999999=10000999988999900001$

11

$1000099999\times 99999999999=100009999898999900001$

12

$1000099999\times 999999999999=1000099998998999900001$