The set of valid inputs for a function is called the the letter a in parentheses above a horizontal line. (a) , and the input variable x is called the the letter b in parentheses above a horizontal line. (b) variable.

  (a)  domain

  (b)  "argument," or "independent variable"

Step-by-step explanation:

You may want to refer to your notes for terminology related to functions. Different terms are used, depending on the context.

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The set of valid inputs for a function is called the domain.

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The input variable x is called the argument, or independent variable.