Answer
a) for all (x,y) b) for all (x,y) except (0,0)
Work Step by Step
a) As we know that $\sin t$ is defined for all real numbers. Therefore, it is defined for all $(x,y)$
b) Since, $\ln t$ is defined for positive real numbers, that is, for all (x,y) except for $(0,0)$.
