连接AE
∠POE=∠OAE ∠OEA=2∠OAE
∠POE=2∠CAB
∠OAE=∠CAB
弧BC=弧BE
AB是直径
CE⊥AB
(2)连接OC
∠P=∠E=∠OCE
∠P ∠PCD=90°
∠P ∠OCE=90°
PC是切线
(3)PC²=PB×PA=9(9 6x)
设OD=x,BD=2x,AB=6x
DC²=2x×4x=8x²
PC²=CD² PD²=8x² (9 2x)²
9(9 6x)=8x² (9 2x)²
x=1.5
半径=3x=4.5
tan∠APC=CD/PD=√2/4
=∠∠∠