\begin{definition}[Definition:Ellipse/Minor Axis/Semi-Minor Axis]
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Consider an ellipse $K$.
A '''semi-minor axis''' of $K$ is either half of the minor axis of $K$ from its midpoint to its endpoint.
In the above diagram, $O C_1$ and $O C_2$ are the '''semi-minor axes''' of $K$.
\end{definition}