cho 2 nguon ket hop cung pha S1,S2 cach nhau 8cm,ve phia S1,S2 lay them 2 diem S3S4 cach nhau 4 cm va hop thanh hing thang can.buoc song la 1 cm.hoi duong cao cua hinh thang lon nhat la bao nhieu de tren S1S2 co 5 diem dao dong voi bien do cuc dai