## Thomas' Calculus 13th Edition

Comparing to Table 12.1, the equation is of the form $\displaystyle \frac{x^{2}}{1^{2}}+\frac{y^{2}}{1^{2}}=\frac{z^{2}}{1}$ this is an elliptical cone, in which the traces in planes parallel to the xy plane ( $z=k$) are circles of radius $k$.