Answer
a) For all $(x,y,z)$ except the plane where $z=0$
b) For all $(x,y,z)$ excluding the surface of the cylinder $x^2+z^2=1$
Work Step by Step
a) We know that $\sin t$ is defined for all real numbers. Since we can not have a zero in the denominator $z \ne 0$.
b) We can not have a zero in the denominator, thus: $x^2+z^2-1\ne 0$. Thus we must exclude the surface of the cylinder $x^2+z^2=1$.
