cho tam giac abc co 3 goc nhon, duong cao ah. Trên nua mat phang bo la duong thang ac co chua diem b, e tia cx song song voi ab,tren tia cx lay diem d sao cho cd bang ab. ke dkvuong goc voi bc. Goi o la trung diem cua bc. Chung minh:
a.ah=dk
b.ba diem a,o,d thang hang
c.ac song song voi bd