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the angle GEF is half a right angle,
and
the angle EGF is right, for it is equal to the interior and opposite angle ECB, [I.29]
the remaining angle EFG is half a right angle; [I.32]
therefore
the angle GEF is equal to the angle EFG,
so that
the side EG is also equal to GF. [I.6]
For the same reason
the side DF is also equal to DB.
