cho tam giac abc vuong tai a (ab<ac), duong cao ah. qua b ve duong thang song song voi ac cat ah tai d. a) chung minh tam giac ahc dong dang voi tam giac dhb. b) chung minh ab^2=ac.bd. c) goi m,n lan luot la trung diem bd, ac. chung minh 3 diem m,h,n thang hang