The number 123456789 is concatenated 1729 times to form a certain number:
A=1729 (123456789)’s123456789123456789…123456789
Similarly, the number 987654321 is concatenated 1729 times to form another certain number:
B=1729 (987654321)’s987654321987654321…987654321
If A+B=C, find the sum of digits of C.
Bonus: Generalize this.