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S		 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,
and
    the angle ADC in the segment ADC less than the semicircle is greater than a right angle.
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