Differences Between Doppler Effect Formulas: u & f´

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u = (v + u') / (1 + v u' / c2)

f´ = f √ [ ( 1 + v / c) / ( 1 – v / c) ]What are the difference between these two formulas? My formula sheet doesn't specify the two. I believe one of them has to do with doppler effect.
 
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First is colinear relatavistic velocity addition. Second is frequency change due to Doppler for a source emitting at f approaching a detector at v; f' is the frequency that will be be measured by the detector.
 
Thank you so much PAllen
 
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