## Calculus (3rd Edition)

$\phi_0=\pi/2$
In spherical coordinates, we have \begin{aligned} &x=\rho \sin \phi \cos \theta\\ &y=\rho \sin \phi \sin \theta\\ &z=\rho \cos \phi \end{aligned} Now, at $\phi_0=\pi/2$; then $z=0$ and hence we get the xy-plane.