# Question #fd964

$A = \left\{x , y \in \mathbb{R} | {x}^{2} + {y}^{2} < 9\right\}$
For the natural logarithm to be defined, what's inside of it must be positive. So, in order for $\ln a$ to exist, $a > 0$ must be true.
$9 - {x}^{2} - {y}^{2} > 0 \implies {x}^{2} + {y}^{2} < 9$.