A source moves away from an obeserver with a certain speed, and the ratio of actual to the apparent frequency as heard by the observer in `eta`. If the two approach each other with the same speed, then find the ratio.

Explanation

`f'=f((v)/(v+v'))`
Where `v'=v_S`
`implies(f)/(f')=eta(given)=1+(v')/(v)` .(i)
`f''=f((v+v')/(v-v'))`
where `v_0=v'`
`implies(f)/(f^''=(v-v')/(v+v')=(1-((v')/(v))/(1+((v')/(v))`
`=(1-(eta-1))/(1+(eta-1))=(2-eta)/(eta)=(2)/(eta)-1`