Proof: By Euclid
(related to Corollary: 4.15: Side of Hexagon Inscribed in a Circle Equals the Radius of that Circle)
This follows immediately from the proof of Prop. 4.15, the definition of a tangent and circumscribing about circle.
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References
Adapted from CC BYSA 3.0 Sources:
 Callahan, Daniel: "Euclidâ€™s 'Elements' Redux" 2014
Adapted from (Public Domain)
 Casey, John: "The First Six Books of the Elements of Euclid"