Calculating Wave Frequency from Source and Receiver Measurements

[grapejellypie]

Homework Statement

A wave source approaches you at constant speed and you measure a wave frequency f1. As the source passes and then recedes, you measure frequency f2. Find the expression for the frequency emitted if the source were stationary in terms of f1, f2, and wave speed v.

Homework Equations

I'm not sure which equations are relevant, but here are some:
f=v/λ (where λ = wavelength)
for situations in which the source is in motion relative to a stationary receiver
fr=(fs)/(1±u/v) where fr=frequency the receiver receives; fs= frequency emitted by source, u=speed of source, and v= wave speed)

The Attempt at a Solution

I'm not sure how to go about solving this problem. I know that the answer will be in terms of variables...
I also know that fs= fr when the source and receiver are stationary

 

[nasu]

You are a little bit confused.
The problem asks to find fs. This is the frequency of the source and this is what the onserver will detect if the source is stationary.
f1 and f2 are the values of fr corresponding to the two signs in your formula for the Doppler effect.