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Book III   Proposition 1
But, when a straight line set up on a straight line makes the adjacent angles equal to one another, each of the equal angles is right [I.Def.10]; therefore
    the angle GDB is right.
But
    the angle FDB is also right;
therefore
    the angle FDB is equal to the angle GDB,
the greater to the less: which is impossible.

Therefore
    G is not the center of the circle ABC.
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