1. The problem statement, all variables and given/known data Two trumpet players are both playing with a pitch of 440 Hz, corresponding to the musical pitch A above middle C. However, one of the trumpet players is marching away from you so that your hear a beat frequency of 4 Hz from the two trumpets. With what speed is the departing trumpet player moving away from you? (The speed of sound in air is 340 m/s) 2. Relevant equations f' = f(V+-Vo/V-+Vs) f2 - f1 = beat frequency 3. The attempt at a solution How I approached the question was the following: f' = f(V/V+Vs) f(V/V+Vs) - f1 = beat so 440(340/340+Vs) - 440 = 4 which solves to Vs = 3.06 m/s However, looking at the answer (3.12 m/s) It seems I should have set the equation up as: 440 - 440(340/340+Vs) = 4 which goes to 3.119 m/s So conceptually speaking, how am I supposed to know how to set up the equation? Is it just what yields a positive answer? or is there something I'm not thinking of. I couldn't find anything specific online. Thanks for the help.