# Solving Laplace transforms from a given, linear graph

• Feb 26th 2010, 02:04 PM
zepto-
Solving Laplace transforms from a given, linear graph
Given a discontinuous linear graph with a range of (0,1), how would go about finding the Laplace transform?
• Feb 26th 2010, 02:10 PM
TheEmptySet
Quote:

Originally Posted by zepto-
Given a discontinuous linear graph with a range of (0,1), how would go about finding the Laplace transform?

How discontinous? if it is a finite set $\displaystyle t_0,t_1,...t_n$
then just break up the integral to get

$\displaystyle \int_{0}^{t_0}e^{-st}f(t)dt+\int_{t_0}^{t_1}e^{-st}f(t)dt+...+\int_{t_{n-1}}^{t_n}e^{-st}f(t)dt$
• Feb 26th 2010, 02:18 PM
zepto-
I think my probelm might be writing the Heaviside / Unit step functions in U(t) format.
• Feb 26th 2010, 02:31 PM
TheEmptySet
Quote:

Originally Posted by zepto-
I think my probelm might be writing the Heaviside functions in U(t) format.

Okay well Here is an example I guess

$\displaystyle f(t)=\begin{cases} e^t, 0 \le t\le 1 \\ e, t > 1\end{cases}$

The Lapalce transform is

$\displaystyle \mathcal{L}(f)=\int_{0}^{\infty}e^{-st}dt=\int_{0}^{1}e^{-st}f(t)dt+\int_{1}^{\infty}e^{-st}f(t)dt$

Now using the definition of f gives

$\displaystyle \mathcal{L}(f)=\int_{0}^{1}e^{-st}e^{t}dt+\int_{1}^{\infty}e^{-st}\cdot 1dt$

Now just evaluate each of these integrals to get

$\displaystyle \mathcal{L}(f)=\int_{0}^{1}e^{-(s-1)t}dt+\int_{1}^{\infty}e^{-st}dt=-\frac{1}{s-1}e^{-(s-1)t}\bigg|_{0}^{1}-\frac{1}{s}e^{-st}\bigg|_{1}^{\infty}=$
$\displaystyle -\frac{e^{-(s-1)}}{s-1}+\frac{1}{s-1}+\frac{1}{s}e^{-s}$
• Feb 26th 2010, 11:28 PM
CaptainBlack
Quote:

Originally Posted by zepto-
Given a discontinuous linear graph with a range of (0,1), how would go about finding the Laplace transform?

Can you post it?

CB