T H E E L E M E N T S	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; therefore     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] therefore each of the angles DCF, FCE is right.
	Previous Page    Return to Propositions    Next Page