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the angles DEG, DEF are also equal to two right angles; [I.13]
therefore
the angles AKB, AMB are equal to the angles DEG, DEF,
of which
the angle AKB is equal to the angle DEG;
therefore
the angle AMB which remains is equal to the angle DEF whcih remains.
Similarly it can be proved that
the angle CNB is also equal to the angle DFE.
