V× nguån nguoc pha nen o la CT.M la cuc dai.vay OM = v/4f.suy v/f = Tren S1S2 co [14,5/2]*2 cuc dai.tuc la co 14 diem dao dong cuc dai Suy co 28 diem dd CD tren elip nhan S1S2 lam tieu diem (vi 1duong CD cat elip tai 2diem nöa trªn vµ díi elip.ok Chän D Cam on vi da doc