A polynomial must be equivalent to an expression with a finite number of
terms all of which are in the form ħAxN where N is a non-negative
whole number, which includes 0, but not negative numbers. A can be any
number, positive, negative or 0.
f(x) =
is not a polynomial because it has a variable
in a denominator. Its numerator is a polynomial and its denominator is
a polynomial, but f(x) itself is not a polynomial.
Even if it is written as
f(x) = 

and then as
f(x) = 

where x is not = 0, its terms are not of the form ħAxN,
where A is any number and N a non-negative whole number.
And even if it is further written as
f(x) = 

where x is not = 0, its terms are not of the form ħAxN,
where A is any number and N a non-negative whole number.
So f(x) cannot be changed into a finite number of terms all of the form
AxN, where A is any number and N a non-negative whole number.
So f(x) is not a polynomial.
Edwin