y=2x10oas40s4-20)]m 2πv=400π， 200Hz u 2 m/s 入=uT=20×。 =0.1m 200 2 = 400,  = 200Hz  1 T = s 200 1 = m 20 2 10 cos 400 3             =  − − x y  t 200 1 = 20 u = 20m/s  = uT = 0.1m