cho tam giac abc co canh ab = ac, m la trung diem cua bc.
1. chung minh tam giac abm = tam giac acm
2. tren tia doi ma lay d sao cho ma = md . chung minh ab// cd
3. tren nua mat phang bo ac ko chua diem b , ve tia ax // bc. lay diem i thuoc ax sao cho ai = bc. chung minh 3 diem d,c,i thang hang