为了能够表述清楚,我把AB延长线上一点为D,AC延长线上一点为E.∠DBC = ∠A + ∠ ACB（外角等于内角和）同理∠ECB = ∠A + ∠ABC两式相加得∠DBC + ∠ECB = 2∠A + ∠ACB + ∠ABC = ∠A + 180º除以2得∠PBC + ∠P...