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Recurring decimals

Students divide fractions out on a long-division board, watch the remainders, and see why decimals either stop or repeat, writing recurring decimals with bars and dots (AC9M8N03, node 1).

Irrational numbers and index laws · Recurring decimals · about 35 minutes · Open the student lesson

Classroom resources

Load these into Waypoints Prepare: choose Load URL and paste the copied address, or download the file and import it. In Prepare, the worksheet's Teacher edition shows the answers; the student copy hides them.

Students will be able to

Timing

Predict and Run 8 min · Investigate 8 min · Modify 10 min · Practise and Make 9 min

Before class

Do the Run board yourself first. Students need fractions as decimals (Number Foundations).

The one idea: the remainders are always less than the denominator, so either one is 0 (the decimal stops) or one comes back (the digits repeat). The common mistake is calling a recurring decimal irrational (Zoe's mistake): a recurring decimal is a fraction.

The board shows the first 12 steps; longer blocks (1/17, 1/19, 1/23, 1/29) are written out in full on the decimal line.

Wider: why 0.999… = 1; the longest blocks (1/7 has 6, 1/17 has 16: the block is at most one less than the denominator).

Running the lesson

  1. Predict: ask a 'yes' student to keep dividing 1 by 3.
  2. Run: ask why the remainder can never be 7 or more when dividing by 7.
  3. Investigate: ask Zoe whether 1/7 is a fraction.
  4. Modify: ask which denominators gave decimals that stop.
  5. Make: ask for the longest block they found.

The hard part (on purpose)

Trusting the remainders over the calculator. Ask students to predict the next remainder before the board shows it.

When students get stuck

What you will seeA question to askThe fix
A student says a long calculator decimal never stops, so it is irrational.Do the digits repeat?A repeating decimal is a fraction: rational.
A student puts the bar over the wrong digits.Which digits come round again?The bar goes over the whole repeating block, and only that.
A student thinks 0.33 is exactly 1/3.What is 3 × 0.33?0.99 is not 1: 0.33 is an approximation.

Answers to the checks

  1. one third: Does the decimal for 1/3 ever stop?
    No. Every step of 1 ÷ 3 leaves a remainder of 1, so every digit is 3, forever. 0.33 is an approximation.
  2. divide it out: Try 1/3, 1/8 and 1/7. Find a fraction whose repeating block is 6 digits long. Was your prediction right?

    1/7 = 0.142857142857…: the remainders 1, 3, 2, 6, 4, 5 come round again.
  3. Notation: How should 0.272727… be written with a bar?
    A bar over 27: 0.27 with the bar over both digits
    The repeating block is 27, so the bar goes over both digits.
  4. Find the mistake: Zoe decides whether 1/7 is a rational number (a fraction). Click the line where her reasoning goes wrong.
    line 4
    Line 4: 1/7 is a fraction, so it is rational. Its decimal never stops, but it repeats (142857). Only a decimal that never stops and never repeats is irrational.
  5. Eleven: Divide 1 by 11 (on paper, or on the Run board above). How many digits are in its repeating block?
    2
    1/11 = 0.090909…: the block 09 repeats.
  6. Stops after 3 places: Find a fraction whose decimal stops after exactly 3 decimal places.

    For example 1/8 = 0.125, or 3/8, 5/8, 1/40.
  7. A delay: Find a fraction with some digits before the repeating block starts.

    For example 1/6 = 0.1666…: the 1 comes first, then the 6 repeats.
  8. Thirteenths: Divide 1 by 13. How long is its repeating block?

    1/13 = 0.076923076923…: a block of 6 digits.
  9. 0.999…: 1/3 = 0.333…, so 3 × 1/3 = 0.999… What does that tell you?
    0.999… is exactly 1
    3 × 1/3 = 1, and 3 × 0.333… = 0.999…, so they are the same number.
  10. Stops or repeats: Which fraction has a terminating decimal?
    3/8
    3/8 = 0.375: it stops.
  11. Sevenths: How many digits are in the repeating block of 1/7?
    6
    1/7 = 0.142857142857…: 6 digits.
  12. Dot notation: How is 0.1666… written with a dot?
    0.16 with a dot over the 6
    Only the 6 repeats: a dot over the 6.
  13. Review: fractions to decimals: Review: write 3/4 as a decimal.
    0.75
    3 ÷ 4 = 0.75.
  14. a two-digit block: Find a fraction with a repeating block of 2 digits and a denominator bigger than 11.

    For example 1/22 = 0.0454545…, or 1/33 = 0.030303… if the board went that far.

Solutions

One possible solution for each task. Other correct programs are fine.

stop, repeat, delay › Stops after 3 places

Open task: any program that does what the step asks.

stop, repeat, delay › A delay

Open task: any program that does what the step asks.

stop, repeat, delay › Thirteenths

Open task: any program that does what the step asks.

stop, repeat, delay › 0.999…

Open task: any program that does what the step asks.

a two-digit block

Open task: any program that does what the step asks.

Stretch and support

Stretch: Find the length of the repeating block of 1/17, 1/19 and 1/23. What is the longest a block can be for denominator b, and why?

Support: Do 1 ÷ 3 and 1 ÷ 4 on paper side by side before the board.

Coach prompt (optional)

For a student who has done the lesson and is still stuck. They paste it into an AI tool your school allows; it asks questions about this lesson's one idea and doesn't give answers. Students see it on the course recap page only through a link ending in ?coach=1: recap with coach prompts.

You are a patient maths coach for a Year 8 student in Australia. Help me understand ONE idea only, from the lesson "Recurring decimals".

The idea: Identifying terminating and recurring decimals: I can Students distinguish terminating from recurring decimals and use correct notation for recurring decimals.

What understanding looks like:
- Correctly classifies a fraction's decimal expansion as terminating or recurring.
- Writes a recurring decimal using correct notation.
- Does not confuse recurring with irrational (non-terminating, non-recurring).

Mistakes students often make here:
- A student says a long calculator decimal never stops, so it is irrational.
- A student puts the bar over the wrong digits.
- A student thinks 0.33 is exactly 1/3.

How to coach me (rules from the Waypoints Kit coach):
- Ask me one short question at a time (2 to 4 sentences) and wait for my answer. Never give me the answer before you have heard my thinking, and don't hide the answer inside your question.
- Start in Discover: ask me to explain the idea in my own words, or to predict what happens in a simple example. I can switch any time: "teach it back" (you play a beginner and I explain), "spot the flaw" (you show me an answer with one common mistake and I find it), or "check" (I explain, and you tell me what is solid and what to revisit).
- If I am stuck or say "just tell me", ask what I think first, then give a small hint or a simpler example. Explain only after I have had a real go.
- Stay in this idea: don't teach later ideas or add extra content. If I ask about something else, say it is for another lesson and bring me back.
- After you help, ask me one reflective question, such as how I know my answer is right or what I would check.
- Praise my effort and method, not my ability.
- You can be wrong: if I rely on something you said, remind me to check it against my notes or the lesson. If you don't know, say so.
- Use simple words.
- Stop when I can explain the idea in my own words and get one new example right. Then tell me to go back to the lesson "Recurring decimals".

Exit ticket

Does 5/6 terminate or recur? Write it with a dot.

It recurs: 5/6 = 0.8333… = 0.83 with a dot over the 3.

What each step is for

Where it leads

This starts AC9M8N03.

Where it leads: predicting from the denominator (next lesson), irrational numbers, and converting recurring decimals to fractions (Year 9 and 10).