\begin{definition}[Definition:Right Circular Cone/Opening Angle]
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Let $K$ be a right circular cone.
Let point $A$ be the apex of $K$.
Let $B$ and $C$ be the endpoints of a diameter of the base of $K$.
Then the angle $\angle BAC$ is the '''opening angle''' of $K$.
In the above diagram, $\phi$ is the '''opening angle''' of the right circular cone depicted.
\end{definition}