Find the addition of the largest 6-digit number and the smallest 7-digit number.

The largest 6-digit number can be written as: 9,99,999

The smallest 7-digit number can be written as: 10,00,000

Addition of these two numbers can be given as:

1 0 0 0 0 0 0
+ 9 9 9 9 9 9
1 9 9 9 9 9 9

Hence the final answer is: 19,99,999.