cho goc nhon xOy tren tia Ox lay hai diem A;B sao cho OA nho hon OB tren tia Oy lay hai diem C;D sao cho OC nho hon OD .Biet OA=OC;OB=OD va AD cat BC tai E

a. chung minh : tam giac EAB= tam giac ECD 

b. chung minh : OE la tia phan giac cua goc xOy 

c. chung minh AC song song voi BD