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    Module 5 Day 2 Your Turn Part 3
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    • I
      InTeReStInG M5 last edited by

      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?
      Thanks!

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      • E
        energizedpanda M2★ M3★ M4 M5★ @InTeReStInG last edited by

        @neatlobster Try to see a pattern on what happens when you subtract 1 from powers of 10. You'll find something neat. ;D

        ~∑nergized Pand∆

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        • I
          InTeReStInG M5 last edited by

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          • I
            InTeReStInG M5 @energizedpanda last edited by

            @energizedpanda
            Hmm, We get 9, 99, 999, .... So they are all mutiples of nine. Ok, now what?
            P.S
            why isn't "9 is equal to 3^2" correct?

            debbie 1 Reply Last reply Reply Quote 3
            • mathnerd_101
              mathnerd_101 M3 last edited by mathnerd_101

              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

              Don't worry, I will die on the AMC10A. It's OK not to be OK!

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              • debbie
                debbie ADMIN M0★ M1 M5 @InTeReStInG last edited by

                @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

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