## Thomas' Calculus 13th Edition

a) for all (x,y,z) b) For all (x,y) except the interior of the cylinder $x^2+y^2=1$
a) As we can see that there are no square roots and fractions , therefore, all values are possible , that is, for all $(x,y,z)$ b) There must be possibility that in the xy plane $x^2+y^2-1 \geq 0$ so, $x^2+y^2 \geq 1$. In order to make sure that the radical does not contain any negative value.