Idea 16of 21Builds onThe rule as 5x + 15
Jump ahead with the rule
How do you find term 20 without writing all the others?
Today you'll discover
- what a term number is
- how a rule jumps straight to a far term
- how to check a prediction
Think firstTerm 0 is 15, term 1 is 20
You want term 20. Which way is quickest and safe?
Decide first, then press Next. Read the term number in, the term value out.
A rule machine beside a table headed term number and term value, terms 0 to 6. The rule, multiply by 5 then add 15, is revealed; the table fills to 45 for term 6; then term 20 is set apart at the bottom with the value 115. The last step shows a rule of times 4 then add 3.
See the whole ideathe big idea and the key word, in one place
The big idea
Use the rule on the term number to jump straight to any term.
Making a predictionSuzie's rule, 5x + 15, predicts her savings in week 20: 5 × 20 + 15 = $115, without a single extra row.
Easy to get wrongDoubling a far term only works when the pattern starts at 0. Term 20 is not twice term 10 here.
Key word
- term number
- a term's place in the pattern: term 0, term 1, term 2 …
Ready to use it?
- Use a pattern rule to find a far term, like term 20 or term 50.
- Explain how you know a prediction is right.