A function is a rule that turns each input into exactly one output. We write f(x) = 2x − 5 for the rule double, then subtract 5. f(1) means the output when the input is 1: f(1) = 2 × 1 − 5 = −3.
A function can be shown as a rule, a table of inputs and outputs, or a graph. If you know an output, you can work backwards to find the input: if f(x) = 9, then 2x − 5 = 9, so x = 7.
Worked example
f(x) = −2x + 4. Find f(5).
- Replace x with 5: −2 × 5 + 4.
- −10 + 4 = −6.
Answer:−6
Key words
- function
- a rule with one output for each input
- input
- the value put in
- output
- the value that comes out
Practise this
New questions every time.
Common Core-referenced: 8.F.A.1