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Suppose F is not the center,
but, if possible,
let G be the center,
and
let GA, GD, GB be joined.
Then, since AD is equal to DB, and DG is common,
the two sides AD, DG are equal to the two sides BD, DG respectively;
and
the base GA is equal to the base GB, for they are radii;
therefore
the angle ADG is equal to the angle GDB. [I.8]
