Idea 17of 21Builds onJump ahead with the rule
Which figure has 41 tiles?
How do you find the term number when you know the value?
Today you'll discover
- how to undo a rule
- which step to undo first
- how to check the answer
Think firstTiles = 2 × figure number + 1
Which figure has exactly 41 tiles?
Decide first, then press Next. Undo the last step of the rule first.
A rule machine, multiply by 2 then add 1, beside a table of figure numbers 1 to 6 and their tiles, 3 to 13. The machine's working and the table fill in; then figure 20 is set apart at the bottom with 41 tiles.
See the whole ideathe big idea and the key word, in one place
The big idea
To find the term number from a value, undo the rule, last step first.
Undo, in reverse order
The rule's last step is undone first.
- tiles41
- − 140
- ÷ 2figure 20
Easy to get wrongCheck by running the rule forwards on your answer: 2 × 20 + 1 = 41.
Key word
- general term
- the rule written with the term number, like 2n + 1, that gives any term
Ready to use it?
- Find which term has a given value.
- Check by putting the term number back into the rule.