## Thomas' Calculus 13th Edition

Dividing with 4, we rewrite the equation as: $\displaystyle \frac{x^{2}}{1^{2}}+\frac{y^{2}}{1^{2}}=\frac{z^{2}}{2^{2}}$ and comparing to Table 12.1, we recognize the form of an Elliptical cone. The cross sections with planes $z=k$ are circles (at $z=\pm 2,$ the radius is 1). Circles are special cases of ellipses.