T H E E L E M E N T S	Book III   Proposition 9
	And since, if in a circle a straight line cut a straight line into two equal parts and at right angles, the center of the circle is on the cutting straight line, [III.1, Por.] the center of the circle is on GK. For the same reason the center of the circle is on HL. And the straight lines GK, HL have no other point common but the point D; therefore the point D is the center of the circle ABC. Being what it was required to prove.
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