Cho goc xOy khac got bet co tia phan giac Ot. Lay diem A thuoc tia Ox, ve tia Am // Oy ( tia Am nam trong goc xOy )
a/ Chung minh goc xAm = goc xOy
b/ Ve tia phan giac An cua goc xAm. Cm: An // Ot
c/ Ve tia Ah vuong goc Ot tai H. Cm AH la tia phan giac cua goc OAm