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The digit sum is the sum of all individual digits in a number. The digital root is the single digit obtained by repeatedly summing digits. These are used in divisibility checks and casting out nines.
공식
Digit sum = sum of all digits; Digital root = 1 + ((n−1) mod 9)
- n
- the number
- d
- digit sum — sum of all digits in n
- r
- digital root — repeatedly sum digits until single digit
단계별 가이드
- 1Digit sum: add all digits together
- 2For 1234: 1+2+3+4 = 10
- 3Digital root: repeat until single digit
- 4Digital root of n = 1 + (n−1) mod 9
풀어진 예시
입력
12345
결과
Digit sum = 1+2+3+4+5 = 15, Digital root = 6
입력
999
결과
Digit sum = 27, Digital root = 9
자주 묻는 질문
What is the divisibility rule for 9?
A number is divisible by 9 if and only if its digit sum is divisible by 9 (or equals 9).
What is "casting out nines"?
An ancient divisibility checking technique using digit sums to detect arithmetic errors.
What is the digital root of a number like 999?
9+9+9 = 27, then 2+7 = 9. The digital root is 9.
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