CHo tam giac ABC can tai A. Goi hai diem D va E lan luot la trung diem cua cac canh AB va Ac, hai doan thang Be va Cd cat nhau tai G
a, CM: AD= AE
b, Tren tia doi cua tia DC ta lay diem K sao cho DK= DG. CM: tam giac ADG= tam giac BDK, tu do suy ra AG song song BK
c, CM: BC + AG > 2*DE