## College Algebra (10th Edition)

RECALL: The denominator of a rational expression is not allowed to be equal to zero since division of zero is undefined. Note that for any real number, the value of $x^2$ will always be zero or higher. This means that the value of $x^2+9$ is 9 or higher. Since the value of the denominator will never be zero, the value of $x$ can be any real number. Thus, no value of the variable will be excluded from its domain.