T H E E L E M E N T S	Book III   Proposition 14
	And the square on AF is equal to the square on CG, for AF is equal to CG; therefore     the square on FE which remains is equal to the square on EG, therefore     EF is equal to EG. But in a circle straight lines are said to be equally distant from the center when the perpendiculars drawn to them from the center are equal; [III.Def.4] therefore AB, CD are equally distant from the center. Being what it was required to prove.
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