What is the frequency of #f(theta)= sin 6 t - cos 21 t #?
The period for both sin kt and cos kt is
The periods for the separate oscillations
Twice the first is seven times the second. This common value
(least) # P = (2pi)/3) is the period for the compounded oscillation f(t).
See how it works.
Note that P/2 used instead of P changes the sign of the second
Frequency is 1/P..