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the two sides FA and AC are equal to the two sides GA and AB, respectively; and they contain a common angle FAG.
Therefore
the base FC is equal to the base GB, and
the triangle AFC is equal to the triangle AGB, and the remaining angles will be equal to the remaining angles respectively, namely those which the equal sides subtend, that is, the angle ACF to the angle ABG, and the angle AFC to the angle AGB. [I.4]
