|Book IV Proposition 12|
Therefore, BK is equal to CK. And, since FB is equal to FC, and FK
the two sides BF, FK are equal to the two sides CF, FK; and the base BK equal to the base CK;
the angle BFK is equal to the angle KFC, [I.8]
the angle BKF to the angle FKC.
the angle BFC is double of the angle KFC, and the angle BKC of the angle FKC.
For the same reason
the angle CFD is also double of the angle CFL, and the angle DLC of the angle FLC.
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