Method of Undetermined Coefficients/Sine and Cosine/Particular Solution

Proof Technique
Consider the nonhomogeneous linear second order ODE with constant coefficients:
 * $(1): \quad y'' + p y' + q y = \map R x$

Let $\map R x$ be a linear combination of sine and cosine:
 * $\map R x = \alpha \sin b x + \beta \cos b x$

The Method of Undetermined Coefficients can be used to find a particular solution to $(1)$ in the following manner.

Method and Proof
Let $\map {y_g} x$ be the general solution to:
 * $y'' + p y' + q y = 0$

From General Solution of Linear 2nd Order ODE from Homogeneous 2nd Order ODE and Particular Solution:
 * $\map {y_g} x + \map {y_p} x$

is the general solution to $(1)$.

It remains to find $\map {y_p} x$.

Let $\map R x = \alpha \sin b x + \beta \cos b x$.

Consider the auxiliary equation to $(1)$:
 * $(2): \quad m^2 + p m + q = 0$

There are two cases which may apply.