证明：①假设PB=PC．∵AB=AC，∴∠ABC=∠ACB．∵PB=PC，∴∠PBC=∠PCB．∴∠ABC-∠PBC=∠ACB-∠PCB，∴∠ABP=∠ACP，在△ABP和△ACP中AB＝AC∠ABP＝∠ACPBP＝CP∴△ABP≌△ACP，∴∠APB=∠APC．这与题目中给定的∠AP...