连接OA,
∵PA是切线,
∴∠PAO=∠PDA=90°,
又∵∠APD=∠OPA,
∴△APD ∽ OPA,
∴
PD
PA =
PA
PO ,
∴PA 2=PD?PO,
又∵PA是切线,
∴PA 2=PB?PC
∴PA 2=PD?PO=PB?PC
又∵∠CPD=∠OPB,
∴△PCD ∽ △POB
∴
PC
CD =
PO
OB =
PO
OC
又△POC ∽ △PBD,则
PO
OC =
PB
BD
∴
PB
BD =
PC
CD .
连接OA,
∵PA是切线,
∴∠PAO=∠PDA=90°,
又∵∠APD=∠OPA,
∴△APD ∽ OPA,
∴
PD
PA =
PA
PO ,
∴PA 2=PD?PO,
又∵PA是切线,
∴PA 2=PB?PC
∴PA 2=PD?PO=PB?PC
又∵∠CPD=∠OPB,
∴△PCD ∽ △POB
∴
PC
CD =
PO
OB =
PO
OC
又△POC ∽ △PBD,则
PO
OC =
PB
BD
∴
PB
BD =
PC
CD .