|Book III Proposition 31|
Let ABCD be a circle and let BC be its diameter, and E its center, and
let BA, AC, AD, DC be joined;
I say that
the angle BAC in the semicircle BAC is right,
the angle ABC in the segment ABC greater than the semicircle is less than a right angle,
the angle ADC in the segment ADC less than the semicircle is greater than a right angle.
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