She works in a university. She's a ____.
读“巴西图”,回答下列问题.
如图,AB为⊙O的直径,C、D为⊙O上不同于A、B的两点,∠ABD=2∠BAC,过点C作CE⊥DB,垂足为点E,直线AB与CE交于点F.
(1)求证:CF为⊙O的切线.
(2)当∠CAB=多少时,从点A、C、F、D为顶点的四边形是菱形.