Problem

A fixed source of sound emitting a certain frequency appears as

f_a

when the observer is approaching the source with speed

v_0

and

f_r

when the observer recedes from the source with the same speed. The frequency of the source is a.

\frac{f_r + f_a}{2}

\frac{f_r - f_a}{2}

\sqrt{f_a f_r}

\frac{2f_r f_a}{f_r + f_a}