证明:
∵PA•PB=PC•PD
∴PA/PD=PC/PB
∵∠APD=∠CPB
∴△PBC∽△PDA
∵PA•PB=PC•PD
∴PA/PC=PD/PB
∵∠CPA=∠BPD
∴△PAC∽△PDB
∴∠PBD=∠PCA
∵∠AOB=∠COD
∴△AOB∽△COD
证明:
∵PA•PB=PC•PD
∴PA/PD=PC/PB
∵∠APD=∠CPB
∴△PBC∽△PDA
∵PA•PB=PC•PD
∴PA/PC=PD/PB
∵∠CPA=∠BPD
∴△PAC∽△PDB
∴∠PBD=∠PCA
∵∠AOB=∠COD
∴△AOB∽△COD