|Book III Proposition 21|
Now, since the angle BFD is at the center,
and the angle BAD at the circumference,
and they have the same circumference BCD as base,
the angle BFD is double the the angle BAD. [III. 20]
For the same reason
the angle BFD is also double the angle BED.
the angle BAD is equal to the angle BED.
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