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Book I   Proposition 5
Accordingly, since
    the whole angle ABG was shown to be equal to the whole angle ACF, of which
    angle CBG is equal to angle BCF,
therefore
    the remaining angle ABC is equal to the remaining angle ACB; and
    they are at the base of triangle ABC.

Of course it was shown that
    angle FBC is equal to angle GCB; and
    they are under the base of triangle ABC.
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