證明:把⊿APB繞點A旋轉(zhuǎn)至⊿ADC的位置(如圖).
則∠ADC=∠APB=∠APC;DC=PB,AD=AP.
∴∠ADP=∠APD.
∴∠CDP=∠CPD(等式性質(zhì))
則PC=DC=PB.