3.
如图,已知△ABC内接于⊙O,AB是直径,OD⊥BC于点D.连接DO并延长到F使AF=OC.
(1)求证:△AOC≌△OAF;
(2)探究:当∠1等于多少度时,四边形OCAF是菱形?请回答并给予证明.
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(1)求证:△AOC≌△OAF;
(2)探究:当∠1等于多少度时,四边形OCAF是菱形?请回答并给予证明.