证明:
∵BD平分∠ABC
∴∠ABD=∠CBD
∵AB=BC,BD=BD
∴△ABD≌△CBD (SAS)
∴∠ADB=∠CDB
∵PM⊥AD,PN⊥CD
∴∠PMD=∠PND
∵PD=PD
∴△PMD≌△PND (AAS)
∴PM=PN