Cdu 52: Mot cai thiTdfc nkm yen doc theo mot true toa cua he quy chieu quan tinh K thi c6 chieu dai la i o- Khi thi/dfe ehuyen dong doc theo true toa vdi toe bkng 0,8 Ian toe anh sang chan khong thi chieu dai eua thtfofc dLfgre he K la: A 0,64