0.9 Repeating

02 May

I got to teach one of my favorite lessons in a Precalculus class this week, which I developed several years ago from a paper by Thomas Osler, Fun with 0.999…

We started with a Quick Poll. Students could select as many or as few choices as they wanted.

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I shared their responses separated

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and grouped together.

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In the first class, one student selected all three choices.

In the second class, 5 students selected all three choices.

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I set the timer for a few minutes and asked students to think individually about how they could argue their selection(s).

Then I asked them to talk together about their ideas.

I walked around and listened. These are the conversations I heard:

A: 1/3 is 0.3 repeating. 2/3 is 0.6 repeating. If we add 1/3 and 2/3, we get 1. If we add 0.3 repeating and 0.6 repeating, we get 0.9 repeating.

B: 1/9 is 0.1 repeating. If we multiply 1/9 by 1, we get 1. If we multiply 0.1 repeating by 9, we get 0.9 repeating.

C: 1/3 is 0.3 repeating. If we add 1/3 three times, we get 1. If we add 0.3 repeating three times, we get 0.9 repeating.

D: If x=0.9 repeating, then 10x=9.9 repeating. (It was clear that a few students had seen Vi Hart talk about 0.9 repeating. Even so, this was all they had for now.)

E: I think this is like Zeno’s Paradox. To walk across the room, you have to walk halfway, and halfway again, and halfway again.

This was the perfect opportunity to deliberately sequence the students’ thinking and let them make connections between their arguments (5 Practices style). With which conversation would you start?

We started with argument C. More than one person shook their head in disbelief, even though they agreed that the argument was convincing.

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Next we moved to argument A, which was very similar to argument C.

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Next we moved to argument B.

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I had a few suggestions of what to do, based on the article from the AMATYC Review. We went to one of those next that the students hadn’t thought of: If x=0.9 repeating, what happens when you divide the equation by 3?

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A student shared their work differently in each class, showing that x=1.

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We moved next to argument D. Again, students shared their thinking differently in each class.

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No one thought about Zeno’s Paradox in the first class. So I asked them how we could express 0.9 repeating as a sum.

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And then I sent a Quick Poll to collect their responses.

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In the second class, I asked the students with argument E to share their thoughts. They got at the infinite sum idea, so without decomposing 0.9 repeating as a class, I sent the Quick Poll. Lots of students came up with a sum that equaled 1. Only one of those was clearly 0.9+0.99 +0.999+…

(I didn’t show them the responses equal to 1 in green when I showed them their results.)

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So we practiced look for and make use of structure together. How can we decompose 0.9 repeating into a sum?

I sent the poll again.

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We concluded the lesson by polling the first question again. In the first class, 4 additional students believed only that 0.9 repeating = 1 at the end.

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In the second class the number of students selecting only choice A changed from 6 to 13.

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Our #AskDontTell journey continues, one lesson at a time …


Posted by on May 2, 2015 in Precalculus


Tags: , ,

8 responses to “0.9 Repeating

  1. Maya Quinn

    May 2, 2015 at 4:54 pm

    Perhaps the article “Does 0.999… Really Equal 1?” from The Mathematics Educator (Norton & Baldwin, 2012) might be worth a look (along with its references):

    • jwilson828

      May 2, 2015 at 5:24 pm

      Thank you for this resource.

  2. howardat58

    May 2, 2015 at 6:09 pm

    Scary stuff! The more you think about repeating decimals the more outrageous the idea is. I liked your student who saw a parallel with Zeno’s Paradox. maybe he/she is ready for binary fractional numbers and will see that 1/2+1/4+1/8+…. is 0.11111…repeating, as a “binary decimal” (should be called binals).

  3. howardat58

    May 2, 2015 at 6:14 pm

    More !
    The only mathematically satisfactory way of coping with repeating decimals is as limits of series.
    It is odd that people are happy to talk about 0.999… as equal to 1, but don’t want to consider 1/2+1/4+1/8+… as equal to 1

  4. Jennie Kelly

    May 4, 2015 at 6:08 pm

    How do you show the quick poll responses grouped?

    • jwilson828

      May 5, 2015 at 5:23 am

      Right-click in the Review workspace & select “Show Student Responses Grouped Together”.

  5. mibonds

    May 6, 2015 at 8:13 am

    Reblogged this on Use More CAS and commented:
    This was a precal class but I think would be an awesome activity with Middle Schoolers.

  6. morethanmaths

    May 10, 2015 at 3:50 am

    Thanks for sharing this – I really like this approach and will be trying it myself next time I teach this. I find that some of my students can see the logic of the various proofs that 0.9 recurring = 1, but in their hearts they aren’t really convinced that the two really are the same. My colleague Phil Routledge suggested a really nice question that I now use a lot with students who experiencing this; “Tell me a number that lies between 0.9 recurring and 1 on the number line?” It’s a really good question that always provokes some interesting discussion.


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