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the angle CAB is equal to the angle BDC, for they are in the same segment BADC; [III. 21]
and
the angle ACB is equal to the angle ADB, for they are in the same segment ADCB; [III. 21]
therefore
the whole angle ADC is equal to the angles BAC, ACB.
Let
the angle ABC be added to each;
therefore
the angles ABC, BAC, ACB are equal to the angles ABC, ADC.
