Cho tam giac ABC can tai A.Tren canh AB lay diem D sao cho AD=BC.Qua D ve 1 duong thang song song voi BC,tren do lay DE=AB(E thuoc nua mat phang chua C,bo la duong thang AB). a) chung minh tam giac EDA=tam giac ABC; b) tinh so do goc DAE?;c) chung minh tam giac ACE deu