Answer
$D=\{(x,y)\in R^2: x^2+y^2\gt 25\}$.
Work Step by Step
Since $f(x,y)=\frac{1}{\sqrt{x^2+y^2-25}}$ is defined for all $x,y$ satisfying that $ x^2+y^2\gt 25$. Hence the domain of $f$ is the region $D=\{(x,y)\in R^2: x^2+y^2\gt 25\}$.
You can help us out by revising, improving and updating this answer.
Update this answerAfter you claim an answer you’ll have 24 hours to send in a draft. An editor will review the submission and either publish your submission or provide feedback.