折四边形ABCD,AB=BC=CD,∠ABC=20°,∠BCD=40°,ABCD,AB = BC = CD , \angle ABC = 20\degree , \angle BCD = 40\degree ,ABCD,AB=BC=CD,∠ABC=20°,∠BCD=40°,
BCBCBC中垂线上取点EEE使得∠AED=90°,\angle AED = 90\degree ,∠AED=90°,求∠BEC.\angle BEC.∠BEC.
60°60\degree60°或150°,150\degree,150°,为什么？？
要几何做法QWQ
