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the angle DAB is equal to the angle ABC,
and
the angle EAB is half the angle DAB,
and
the angle EBA half the angle ABC,
therefore
the angle EAB is also equal to the angle ABC;
so that
the side EA is also equal to EB. [I.6]
Similarly we can prove that
each of the straight lines EA, EB is equal to each of the straight lines EC, ED.
