tam giac AHE dong danh tam giac AHB suy ra AH/HB=AE/AB=AF/BC (1) goc ABH+HAB=90 va ABH+HBC =90 suy ra HAF=CHB (2)(1) va (2) suy ra tam giac AHF dong dang tam giac BHC suy ra goc AHF= goc BHC ma goc AHF+ goc FHB =90 suy ra goc FHB+goc BHC =90 suy ra HF vuong goc voi HC suy ra viet dc pt HC:x+y=0 suy ra HC giao d =C(-1/3;1/3)may cai ki hieu viet no ko ra nen viet loi thoi \Rightarrow
tam giac AHE dong dang tam giac AHB suy ra AH/HB=AE/AB=AF/BC (1) goc ABH+HAB=90 va ABH+HBC =90 suy ra HAF=CHB (2)(1) va (2) suy ra tam giac AHF dong dang tam giac BHC suy ra goc AHF= goc BHC ma goc AHF+ goc FHB =90 suy ra goc FHB+goc BHC =90 suy ra HF vuong goc voi HC suy ra viet dc pt HC:x+y=0 suy ra HC giao d =C(-1/3;1/3)may cai ki hieu viet no ko ra nen viet loi thoi \Rightarrow