Definition
Function rule
A rule that assigns exactly one output to each allowed input.
What it means
Send an input through the rule and read the output. The rule may be shown in words, a table, a diagram, or a formula, but the same input cannot be sent to two different outputs.
Remember it
One input goes through one rule and lands on one output.
Example
Use the rule “multiply by 3, then subtract 2.” Input 4 gives 4 × 3 − 2 = 10. Input 7 gives 19. Each input receives one output.
Watch for
A rule that sends input 5 to both 12 and 20 is not a function rule, because one input has two outputs.
Common mix-up
A pattern can suggest a rule; a function rule must determine one output for every allowed input.
Exact meaning
A function rule is a stated process or relationship that assigns exactly one output to each allowed input.