It is always possible to write a sum of sinusoidal functions f(θ) = a cos θ + b sin θ as a single sinusoid the form f(θ) = c cos(θ + δ). This can be done by expanding (-1) using the trigonometric addition formulas to obtain f(θ) = c cos θ cos δ - c sin θ sin δ.

Fourier series | prosthaphaeresis formulas | simple harmonic motion | sinusoid | trigonometric addition formulas | trigonometry