T H E E L E M E N T S	Book III   Proposition 17
	For, since E is the center of the circles BCD, AFG,     EA is equal to EF, and ED to EB; therefore     the two sides AE, EB are equal to the two sides FE, ED: and they contain a common angle, the angle at E; therefore     the base DF is equal to the base BA, and the triangle EDF is equal to the triangle EBA, and the remaining angles to the remaining angles; [I.4] therefore the angle EDF is equal to the angle EBA. But     the angle EDF is right; therefore     the angle EBA is also right.
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