|Book IV Proposition 9|
Now, since DA is equal to AB, and AC is common, therefore
the two sides DA, AC are equal to the two sides BA, AC; and the base DE is equal to the base BC;
the angle DAC is equal to the angle BAC. [I.8]
the angle DAB is bisected by AC.
we can prove that each of the angles ABC, BCD, CAD is bisected by the straight lines AC, DB.
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