This may be a simple question and I am not a mathematician. I recently noticed that 11+12+13 = 36 whose digit sum is 9. And that the digit sum 1+1+1+2+1+3 also equals 9. Is there a pattern here that can be extrapolated to other addends?
2026-03-27 20:21:11.1774642871
Digit sum of the sum equals digit sum of the addends
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The digit sums may differ: $99+2=101$ has digit sum $1+0+1=2$, but on the left we have $9+9+2=20$.
However, there is a pattern. The digit sum of a number differs from the number itself by a multiple of $9$ (that is why (repeated) digit sums can be used to check divisibility by $9$). Therefore the digit sum of the sum also differs by a multiple of $9$ form the digit sum of the summands. (So above: The difference between $20$ and $2$ is $18=2\cdot 9$)