cho diem o tren duong thang xy . tren duong thang do dat cac doan oa=2 cm,ob=3cm , roi lay diem e va f sao cho a la trung diem cua doan thang oe, b la trung diem cua doan thang of.tinh do dai doan ef
( sorry cac bn nhe may tinh cua mk ko danh duoc dau nhung chac cac bn dich ra duoc. thanh that xl nhe )