If you have sines and cosines of some fundamental frequency, and you’re able to take products and sums, then you can construct sines and cosines at any multiple of the fundamental frequency.
Here’s a proof.
Taking real parts gives us cos nθ in the first equation and the even terms of the sum in the last equation.
Taking imaginary parts gives us sin nθ in the first equation and the odd terms of the sum in the last equation.
A radio may use this technique to create signals with frequencies higher than the frequency of its oscillator.
Read more on mathematics and radio.