I was just reviewing the Doppler effect, and I thought of something :
the formula involves (v-vr)/(v-vs)
let's say v = 300
vr = 30
vs = 0 (stationary observer)
it will be equal to 270/300
however, if we take the opposite, i.e. consider the reference frame to be that the source is moving not the observer we get
330/300
The 2 are obviously not equal, so is the formula just an approximation?