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RE: Brainsteem Mathematics Challenges - Digital Sum
I think the sum of digits of every factorial after 5! is 9. Is this correct? I think this works as long as you continue the process. I remember from Number Theory that mod 9 and factorials are friends. I forget and I want to know why! Thanks :)
It's not clear to me that you are supposed to keep summing. So, the sum of the digits in 15! is 45, and I don't think you are to add those together until you get to a one digit number. If you do, you are correct - they all sum to 9.
yeah, a bit of clarification on definitions.
The digital sum is the sum of all the digits.
The digital root is the continued digital sum until you get to a single digit. So, yes, the digital root of n! (n>5) will always be 9.