Could anyone please help? How do you get the answer to 3 to the 10000th power (3^10000)?

Problem:

After figuring out 3^10000, I need to add up all its digits and thus obtain a new number. Then I need to add up the digits of this new numbers and obtain another number. I need to continue doing this until eventually I get a single digit number.

Do I have to know the full set of 3^10000 number to end up with the single digit number after the above process?

thanks for helping.

hongvo