## Thomas' Calculus 13th Edition

a) For all (x,y,z) except the plane where $z=0$; b) For all $(x,y,z)$ excluding the surface of the cylinder $x^2+z^2=1$
a) Since, $\sin t$ is defined for all real numbers. This means that there must not be zero in the denominator. b) There must not be zero in the denominator, therefore, we must have $x^2+z^2-1 \ne 0$ and excluding the surface of the cylinder $x^2+z^2=1$