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Many Doppler effect questions require to find the final Frequency/Wavelength. so after using the equation :

Δλ / λ = v / c or Δf / f = v / c

How can I know if the change in Frequency/Wavelength is positive or negative to calculate the final frequency or wavelength. because sometimes they add the change to initial but in others they subtract .

i know this's all related to :

* Δf= f

_{f}- f

_{i }or Δλ=λ

_{f}- λ

_{i}

* Source approaching: frequency increase , hence wavelength gets shorter

* Source moving away : frequency decrease , hence wavelength gets longer