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Book I   Proposition 5
Accordingly, since
    the angle ABG was proved equal to the angle ACF,
and in these
    the angle CBG is equal to the angle BCF,
    the remaining angle ABC is equal to the remaining angle ACB;
and they are at the base of the triangle ABC.

But
    the angle FBC was also proved equal to the angle GCB;
and they are under the base.

Being what is was required to prove.
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