Kaplan Doppler Question

I don't understand their reason for using the doppler equation in the form that they did. In this case, the sound SOURCE and the sound DETECTOR are the same, so I believe we need to use a different equation.

In Khan Academy's video, they derive a different equation for when the source & the detector are the same, such as in this example. The formula they arrive at is in the green box



can someone explain when I'm...

Kaplan Doppler Question