cho hai tia oa, ob khong doi nhau. Lay cac diem A va B khong trung o sao cho A thuoc tia oa,Bthuoc tia ob, Goi C la diem nam giua A,B. Ve diem D sao cho B nam giua Ava D. Hoi trong hai tia oc,oD thi tia nao nam giua hai tia OA,OB; tia nao khong nam giua hai tia oa, ob.