Primitive of Exponential of a x by Cosine of b x/Proof 3

Proof
Let $a, b, x \in \R$ be real numbers.

Suppose $a \ne 0 \ne b$.

Denote $\ds f_1 = \map \exp {a x} \map \cos {b x}$, $f_2 = \map \exp {a x} \map \sin {b x}$.

Let $\map \CC \R$ be the space of continuous real-valued functions.

Let $\struct {\map {\CC^1} \R, +, \, \cdot \,}_\R$ be the vector space of continuously differentiable real-valued functions.

Let $S = \span \set {f_1, f_2} \subset \map {\CC^1} \R$ be a vector space.

Let $D : S \to S$ be the derivative $x$.

From Differentiation of Exponential of a x by Cosine of b x and Exponential of a x by Sine of b x wrt x as Invertible Matrix, $D$ is expressible as:


 * $\mathbf D = \begin{bmatrix}

a & b \\ -b & a \end{bmatrix}$

and is invertible.

By Inverse of Matrix is Scalar Product of Adjugate by Reciprocal of Determinant:


 * $\ds \mathbf D^{-1} = \frac 1 {a^2 + b^2} \begin{bmatrix}

a & -b \\ b & a \end{bmatrix}$

Then:


 * $\ds \mathbf D^{-1} \begin{bmatrix} 1 \\ 0 \end {bmatrix} = \frac 1 {a^2 + b^2} \begin{bmatrix}

a & -b \\ b & a \end{bmatrix} \begin{bmatrix} 1 \\ 0 \end{bmatrix} = \frac 1 {a^2 + b^2} \begin{bmatrix} a \\ b \end{bmatrix}$

Application of $\mathbf D$ on both sides on the left and writing out explicitly in terms of $f_1$ and $f_2$ yields:


 * $f_1 = \ds \dfrac \d {\d x} \frac {a f_1 + b f_2} {a^2 + b^2}$

Integrate $x$:


 * $\ds \int f_1 \rd x = \frac {a f_1 + b f_2} {a^2 + b^2} + C$

where $C$ is an arbitrary constant.

Substitute definitions of $f_1$ and $f_2$ to get the desired result.