YLL討論網

過A作兩圓的外公切線XY(令X在A的左邊Y在右邊)
連接AD
∠YAD=∠CDA
∠YAC=∠ABC
∠CDA=∠ABC+∠DAB
∠YAD=∠YAC+∠DAC
∠DAB=∠DAC
AD平分∠BAC

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