== Fundamentals of Laplace Transform ==
Let the signal be:
$ x(t) =e^ {-at} \mathit{u} (t) $
On doin a Laplace Transform
$ X(s)= \int_{-\infty}^{\infty}x(t){e^{-st}}\, dt $
$ X(s)= \int_{-\infty}^{\infty}{e^{-at}}{e^{-st}dt \mathit{u} (t) =1 t>0 $
