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the angle BCD is equal to the angle CDE,
and
the angle FCD is half of the angle BCD,
and
the angle CDF half of the angle CDE,
therefore
the angle FCD is also equal to the angle CDF;
so that
the side FC is also equal to the side FD. [I.6]
Similarly it can be proved that
each of the straight lines FB, FA, FE is equal to each of the straight lines FC, FD.
