Doppler ultrasound is often used to measure the flow rate of blood. It is assumed that the relationship between red blood cells and ultrasound scattering can be shown as R = Bf^4, where R is the fraction of scattering back to the probe, B is the proportionality constant, and f is Ultrasonic frequency. The penetration of ultrasound in tissue is p = e^-aft, where p is the fraction of penetration, a is the attenuation coefficient of the tissue, and t is the thickness of the tissue. Prove that when the frequency f=2/(at), the maximum echo signal can be obtained.