Laplace Transform of Cosine/Proof 4

Proof
By definition of the Laplace Transform:


 * $\displaystyle \laptrans {\cos at} = \int_0^{\to +\infty} e^{-s t} \cos at \rd t$

From Integration by Parts:


 * $\displaystyle \int f g' \rd t = f g - \int f'g \rd t$

Here:

So:

Consider:


 * $\displaystyle \int e^{-s t} \sin a t \rd t$

Again, using Integration by Parts:


 * $\displaystyle \int h j \,' \rd t = h j - \int h'j \rd t$

Here:

So:

Substituting this into $(1)$:

Evaluating at $t = 0$ and $t \to +\infty$: