Texas Instruments TI-89 Titanium User Manual page 215

Arguments (x and y in these examples) are placeholders that represent whatever values
you pass to the function. They do not represent the variables x and y unless you
specifically pass x and y as the arguments when you evaluate the function.
Creating a User-Defined Function
Use one of the following methods.
Method
§
Define command
Program Editor
Calculator Home Screen
Description
Store an expression to a function name (including
the argument list).
Define a function name (including the argument
list) as an expression.
Refer to Programming or information on creating a
user-defined function.
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