\begin{definition}[Definition:Conic Section/Intersection with Cone/Ellipse]
{{:Definition:Conic Section/Intersection with Cone}}
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Let $\theta < \phi < \dfrac \pi 2$.
That is, the angle between $D$ and the axis of $C$ is between that for which $K$ is a circle and that which $K$ is a parabola.
Then $K$ is an ellipse.
\end{definition}