Idea 7of 23Builds on3n means 3 times n
Put a number in for the letter
How do you find the value of an expression?
Today you'll discover
- what substituting means
- the order to work it out
- why 3n + 2 is 17 when n = 5
Think first3n + 2, when each bag holds 5
If n = 5, what is 3n + 2?
Decide first, then press Next. Put the number in, then multiply before you add.
Three bags marked n and 2 loose squares, with 3n + 2 under them. Then each bag opens to show 5, with n = 5, and the working 3 × 5 + 2 = 15 + 2 = 17. The last step shows five bags of n beads with 4 taken out: 5n − 4.
See the whole ideathe big idea and the key word, in one place
The big idea
To evaluate, put the number in for the letter, then work it out.
Step by step
Substitute, multiply, then add.
- start3n + 2
- n = 53 × 5 + 2
- multiply15 + 2
- add17
Easy to get wrong3n with n = 5 is 3 × 5 = 15, never 35.
Key words
- evaluate
- work out the value of an expression for a given number
- substitute
- put a number in place of a letter
Ready to use it?
- Evaluate an expression for a given value of its letter.