Question

三角形ABC是圆O的内接三角形,AB=AC,BD是圆O的直径,连接AD并延长交BC的延长线于点E,

Answer 1

过A作AF⊥BC于F,

∵AB=AC,∴BF=CF,

∴AF垂直平分BC,

∴圆心O在直线AF上,

∵AP∥BC,

∴AF⊥AP,

∴PA为圆的切线.