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DE is equal to EG, and EF is common,
the two sides DE, EF are equal to the two sides GE, EF;
and
the base DF is equal to the base FG;
therefore
the angle DEF is equal to the angle GEF, [I.8]
and
the triangle DEF is equal to the triangle GEF,
and
the remaining angles are equal to the remaining angles, namely those which the equal sides subtend. [I.4]
