|Book III Proposition 32|
ABCD is a quadrilateral in a circle,
its opposite angles are equal to two right angles. [III.22]
the angles DBF, DBE are also equal to two right angles;
the angles DBF, DBE are equal to the angles BAD, BCD,
the angle BAD was proved equal to the angle DBF;
the angle DBE which remains is equal to the angle DCB in the alternate segment DCB of the circle.
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