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Let the angle ABC be added to each; therefore
the whole angle ABD is equal to the whole angle FBC. [C.N.2]
And, since
AB is equal to FB, and BD to BC,
the two sides AB, BD are equal to the two sides FB, BC respectively, and
the angle ABD is equal to the angle FBC;
therefore
the base AD is equal to the base FC,
and
the triangle ABD is equal to the triangle FBC. [I.4]
