# How they get to the 3rd equation

I don't understand hLink how they get to the 3rd equation... I'm completely lost...

## Answers and Replies

mathman
Science Advisor
(f-f0)/f0=(1-(v/c)2/2)/(1-v/c)-1
(f=frequency, v=velocity)

Simplify right hand side to get:(v/c-(v/c)2/2)/(1-v/c)

Assume v<<c, you end up with v/c.