# Question #608f4

##### 1 Answer
Feb 23, 2017

By default, the domain is everywhere that function is defined.
And this function is just undefined when the denominator is equal to zero, so we have to remove all of the values that make our denominator zero from Real numbers set.

So we have to figure out when the denominator is zero.
${x}^{2} - 81 = 0 \implies {x}^{2} = 81 \implies x = \pm 9$
And our domain will be:
$D = \setminus R - \left\{9 , - 9\right\}$