B1:Cho tam giac ABC co AB=AC.Tren cac canh AB,AC lan luot lay cac diem D va E sao cho AD=AE.Cac doan thang BE va CD cat nhau tai K.CM rang: a.Tam giac ABE va tam giac ACD = nhau
b.Tam giac KBD va tam giac KCE = nhau
c.Goi I la trung diem cua BC. CM cac diem A,K,I thang hang