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if the angle ABC is unequal to the angle DEF, one of them is greater. Let the angle ABC be greater; and
on the straight line AB, and at the point B on it,
let the angle ABG be constructed equal to the angle DEF. [I.23]
Then, since
the angle A is equal to the angle D,
and
the angle ABG to the angle DEF;
therefore
the remaining angle AGB is equal to the remaining angle DFE. [I.32]
