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S		 Book I   Proposition 32
The whole angle ACD is equal to the two interior and opposite angles CAB, ABC.

    Let the angle BCA be added to each;
therefore
    the angles BCA, ACD are equal to the three angles CAB, ABC, BCA.

But
    the angles BCA, ACD are equal to two right angles; [Prop.13]
therefore
    the angles CAB, ABC, BCA are also equal to two right angles.
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