Answer
set of all real numbers
Work Step by Step
In a fraction, the denominator cannot be $0$.
Note that $x^2\geq0$ for all real numbers.
Hence, $x^2+1\geq1$ for all real numbers. This means that the denominator will never be $0$.
Therefore, there are no numbers for which the function is undefined and so the domain is the set of all real numbers.
