Calculus (3rd Edition)

We have a solid sphere of radius less than or equal to $1$.
Since $$x=r\cos \theta,\quad y=r\cos \theta, \quad z=z,$$ then the set $$x^2+y^2+z^2\leq1$$ takes the form $$r^2\cos^2\theta+r^2\sin^2\theta+z^2=r^2+z^2\leq1$$ which is a solid sphere of radius less than or equal to $1$.