] gives True if expr is a numeric quantity, and False otherwise.
An expression is considered a numeric quantity if it is either an explicit number or a mathematical constant such as Pi, or is a function that has attribute NumericFunction and all of whose arguments are numeric quantities.
In most cases, NumericQ[
] gives True whenever N[
] would yield an explicit number.
See the Mathematica book: Section 2.3.5, Section 3.1.2.
See also: NumberQ.
Because Pi and E are numeric, you can operate with them much as you can with other numbers.
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