[填空题]
The Doppler effect usinlv6 gof0w qx3+i7l f w8m/hgce:u- eu6g ujfpydzp :95p2s/jh bbsqb0*:ltrasonic waves of frequency $2.25 \times10^{ 6}$Hz is used to monitor the heartbeat of a fetus. A (maximum) beat frequency of 500 Hz is observed. Assuming that the speed of sound in tissue is $1.54 \times10^{3 }$ m/s calculate the maximum velocity of the surface of the beating heart. m/s
