## Calculus (3rd Edition)

(a) If $\phi=0$, then we get $x=0$, $y=0$ and $z=\rho$. So the point lies on the z-axis and the statement is correct. (b) If $\phi=0$, then we get $x=0$, $y=0$ and $z=\rho$. So the point does not lie on the xy-plane and the statement is not correct.