#### Answer

This expression is defined for all real numbers.

#### Work Step by Step

A rational expression is undefined when the value of its denominator is zero. Write and solve an expression where the denominator is zero.
$x^2+1=0$
Use the addition property of equality to solve both equations for y.
$x^2+1-1=0-1$
Simplify.
$x^2=-1$
Since the square of a number can never be negative, this denominator can never be negative. This expression is defined for all real numbers.