A person who was listening to a siren reported that the frequency of the sound fluctuated with time, measured in seconds. The minimum frequency that the person heard was 400 Hz, and the maximum frequency was 1100 Hz. The maximum frequency occurred at t=0 and t=12. The person also reported that, in 12 seconds, she heard the maximum frequency 4 times (including the times at t=0 and t=12). What is the equation of the cosine function that describes the frequency of this siren?