Solution
= Solution
Put $R=|b|$ so that the radial range respects the stated allowance $b\ne0$. With the azimuth measured from the starting meridian, the <cylindrical coordinates> of the curved <surface> satisfy
$$
\boxed{0\leq r\leq R,\qquad0\leq\theta\leq\alpha,\qquad
z=a\left(1-\frac{r^2}{b^2}\right)}.
$$
At $r=0$ all azimuths describe the same apex. The <surface> is a sector of an <elliptic paraboloid>.