Create a cosine function in the form $f(x)=a·cos(fx-o)+c$.

Input parameters:

1. a → Amplitude of cosine wave.
2. f → Frequency of cosine wave, as a multiple of $2\pi$.
3. o → Horizontal offset of cosine wave.
4. c → Vertical offset of cosine wave.

Output parameters:

1. Fn → Cosine function in the form $f(x)=a·cos(fx-o)+c$.