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let AE be joined and drawn through to F.
Then, since
EA is equal to EB,
the angle EAB is also equal to the angle EBA; [I.5]
therefore
the angles EAB, EBA are double of the angle EAB.
But
the angle BEF is equal to the angles EAB, EBA; [I.32]
therefore
the angle BEF is also double of the angle EAB.
