Divisibility by 9 with negative number

Question

I know the rule to check divisibility by 9: check if the sum of the digits of the number is divisible by 9. But what if the number is negative?

Thanks in advance!

Answer 1

Multiplying by -1 doesn't change anything. If we have 18, which we know is divisible by 9, we just multiply it by -1 to get -18 and then we do the same to how many times 9 goes into 18 to get -2, and see that 9 goes into -18 -2 times, -2 is an integer so the rule holds up.