T
H
E

E
L
E
M
E
N
T
S
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,
therefore
    the angle BFD is double the the angle BAD. [III. 20]
For the same reason
    the angle BFD is also double the angle BED.

Therefore
    the angle BAD is equal to the angle BED.
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