Answer
a) For all (x,y,z) except the plane $(x,0,0)$ -- that is, the $x$-axis
b) For all $(x,y,z )$ except the plane $(0,y,0)$ and $(x,0,0)$ -- that is, excluding the $x$ and $y$ axes
Work Step by Step
a) To avoid a zero in the denominator, $y$ and $z$ can not both be zero. Thus, all (x,y,z) except the plane (x,0,0).
b) To avoid a zero in the denominator, we can not have $z$ and $x$ both be zero; similarly, we can not have $z$ and $y$ both be zero. Thus, for all $(x,y,z)$, except the plane $(0,y,0)$ and $(x,0,0)$.
