It says that "See the video for the explanation, but I have checked multiple times and I don't get why powers of ten have to do with the divisibility rule for 3 and 9. Can someone please help?
@neatlobster Try to see a pattern on what happens when you subtract 1 from powers of 10. You'll find something neat. ;D
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Hmm, We get 9, 99, 999, .... So they are all mutiples of nine. Ok, now what?
why isn't "9 is equal to 3^2" correct?
I haven't taken the course, but to prove the divisibility of 9 rule, basically what you do is write out the number in base ten form, or just like: a_110^n+a_210^(n-1)...+a_n10^0. So, we can take this modulo 9 and we get a_1+a_2...+a_n. So if a_1+a_2...+a_n is divisible by 9, then a_110^n+a_210^(n-1)...+a_n10^0 is divisible and our proof is complete.
EDIT: The reason this has to do with powers of ten is because we write it out with powers of ten
@neatlobster Please take a look at your other post where I answered your question. https://forum.poshenloh.com/topic/707/question-what-does-it-mean-by-then-the-value-of-a-digit-in-its-place-is-exactly-the-sum-you-get-when-you-add-up-these-stray-1-s