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the angle BDA is equal to the angle CBD, since the side AD is also equal to AB; [I.5]
so that the angle DBA is also equal to the angle BCD.
Therefore
the three angles BDA, DBA, BCD are equal to one another.
And, since
the angle DBC is equal to the angle BCD,
the side BD is also equal to the side DC. [I.6]
But
BD is by hypothesis equal to CA;
therefore
CA is also equal to CD,
so that
the angle CDA is also equal to the angle DAC.
