Method of Undetermined Coefficients/Sine and Cosine/Particular Solution/i b is Root of Auxiliary Equation

Proof Technique
Consider the nonhomogeneous linear second order ODE with constant coefficients:
 * $(3): \quad y'' + b^2 y = \alpha \sin b x + \beta \cos b x$

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:
 * $y'' + b^2 y = 0$

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 $\map {y_p} x$.

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

We have that $i b$ is a root of $(2)$.