|Book I Proposition 11|
For, since DC is equal to EC, and CF is common
(to the triangles DCF and ECF),
the two sides DC, CF are equal to the two sides EC, CF respectively; and the base DF is equal to the base EF;
the angle DCF is equal to the angle ECF; [I.8]
and they are adjacent angles.
But, when a straight line set up on a straight line makes the adjacent angles equal to one another,each of the equal angles is right; [Def.10]
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