Doppler Shift Equation: How to Calculate Frequency of a Stationary Train

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To calculate the frequency detected by a stationary observer from a moving train, the Doppler Shift equation is used: f' = f(V + Vd) / (V - Vs). In this scenario, the train moves toward the detector at 31 m/s and emits a 305-Hz sound. The expected detected frequency is 340 Hz, but confusion arises regarding the correct application of the formula. The alternative formula provided, f' = f(1 / (1 - (v/v_s))), is also relevant for a source approaching the detector, where v_s represents the speed of sound in air. Understanding these equations is crucial for accurately determining the perceived frequency.
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A train moving toward a detector at 31m/s blows a 305-Hz horn. What frequency is detected by a statiory train?

Using the equation f´= f(V + Vd/ V - Vs)

The answer is supposed to be 340-Hz but I don't understand how to get it. Mind helping me out?
 
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anyone know?
 
I do not recognize the formula. My scientific encyclopedia gives this for the source approaching the detector:

f' = f(\frac{1}{1-\frac{v}{v_s}})

where v_s is the speed of sound in air.
 

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