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Book IV   Proposition 10
Therefore
    the angles CDA, DAC are double of the angle DAC.
But
    the angle BCD is equal to the angles CDA, DAC;
therefore
    the angle BCD is also double of the angle CAD.
But
    the angle BCD is equal to each of the angles BDA, DBA;
therefore
    each of the angles BDA, DBA is also double of the angle DAB.
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