∠D=180-∠DBC-∠DCB∠DBC=∠DBA180-∠DBA=∠A+AEB∠AEB=DEC∠D=∠A+180-∠D-∠DCF-∠DCB∠DCF+∠DCB=180∠D=∠A+180-∠D-180所以可以得2∠D=∠A
