因为由平分线得∠A+∠B=∠ACD,又由平分线得∠ACD=∠A1CD,∠ABA1=∠A1BC所以2∠A1CD=∠A+2∠A1BC因为∠A1CD=∠A1CD+∠A1即2∠A1CD=2∠A1BC+2∠A1所以∠A1=(2∠A1CD-2∠A1BC)÷2∠A1=∠A1CD-∠A1BC以此得出规律计算∠n=∠nCD-=∠nBC