theo toi nghi bai nay rac roi o cho :"khoang thoi gian hai lan lien tiep trung diem P cua doan MN co cung li do voi diem M la 0.1s" .Khong biet tui lam co dung hay ko nhung tui cung xin dua ra 1 huong giai quyet cho bai toan nay. Ko biet Anh DUc co biet cong thuc nay ko : T=t/N , voi N la so lan dao dong. Gio ta xet "khoang thoi .... 0.1s". ban hay chu y bay gio tui se tach va phan tich tung ve cho ban de hieu :"khoang cach giua hai lan lien tiep" se =0.1s nghia la "2T=t""2T=0.1" (giai thich them :cu moi lan di qua diem P ta duoc 1 chu ki, o day de bao la '2 lan lien tiep' nen ta co 2T).                                 Ta co 2T=0.1 va T=t/N nen suy ra N=2 , suy ra T=0.05s                         Van de thu hai :"diem bung M cach nut gan nhat N 1 doan 10cm".              Ta da biet khoang cach giua 1 bung va 1 nut lien tiep la /4.                       Vi ta xet o diem P,ma diem P la trung diem doan MN nen x=5cm . Suy ra : x=5=/4 suy ra =20 vay v=20/0.05=400cm/s . dap an A.                     Tui khong biet cach lam nay co dung hay khong . Phien ban cho xin dap an dung . Vi toi chua chac chan ve cach lam nay lam . Toi con co huong giai quyet khac voi dap an la 100cm/s:nghia la 2=10 suy ra=5 suy ra v=100 nhung tui nghi chac cach nay khong kha quan lam . xin cho y kien . Tom lai ban dua ra nhung bai toan kha hay day, mong ban dua them de cung hoc . cam on .