Question

如图,在△BCE中,EB=EC,∠E=40°,A,D分别在EB、EC上,且AD∥BC,AB=BC,求∠ACD的度数.

Answer 1

∵AB＝BC,∴∠ACB＝∠BAC＝∠E＋∠ACE,∵EB＝EC,∴∠B＝∠BCE＝∠E＋2∠ACE,在△BCE中,∠B＋∠E＋∠BCE＝（∠E＋2∠ACE）＋∠E＋（∠E＋2∠ACE）＝3∠E＋4∠ACE＝3×40°＋4∠ACE＝180°解之得：∠ACE＝15°