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如图,PAB、PCD是圆O的两条割线,AB是圆O的直径,AC//OD.
(1)求证:CD=_________(先填后证).
(2
(2006,南充)如图,PAB、PCD是⊙O的两条割线,AB是⊙O的直径,AC∥OD.
(1)求证:CD=________(先填后证);
(2)若,试求的值.