连接AE
假设角B+角C+角D不等于360°
因为AB平行ED
所以角A+角E=180°
则角A+角E+角B+角C+角D不等于540°
又因为角A+角E+角B+角C+角D等于540
矛盾
所以角B+角C+角D等于360°