## Calculus (3rd Edition)

$D= \{ (x,y) \in \mathbb{R}^2 : -9\leq x\leq 9\}$
Given $$f(x,y)= \sqrt{81-x^2}$$ Since $f(x,y)$ is defined for \begin{align*} 81-x^2\leq 0\\ x^2 \leq 81\\ -9\leq x\leq 9 \end{align*}