Indefinite Integrals / Laplace Transforms

[EvanQ]

Homework Statement

Using improper integrals, find the Laplace transform of f(t)=t, determining the values of s for which the transform is valid.

Homework Equations

The Laplace transform F(s) of a function f is defined as
∞
/
F(s)= | f(t)e^(-st) dt.
/
0

The Attempt at a Solution

I have completely worked the question up until, and am pretty sure I know how to finish it off after integrating the Laplace transform function. It's been hinted at me to use integration by parts but I am completely lost at how to do this.

Please help out?

 

[Dick]

I'll give you a starting point. integral(t*exp(-s*t)*dt)=(-1/s)integral(t*d(exp(-s*t)). Now can you do the integration by parts on the parts t and exp(-s*t)?

 

[Astronuc]

Generally for integration by parts -
$$\int_a^b f(x) g'(x)\,dx\,=\,\left[ f(x) g(x) \right]_{a}^{b} - \int_a^b f'(x) g(x)\,dx$$

let f(x) = t and g'(x) = e^-st.

 

[EvanQ]

mmm thanks for the help guys, but it seems my problem was just a complete lack of knowledge on the topic, and even your help is going over my head.
looks like i'll just flunk this question and hit the books so it doesn't happen again.

 

[Dick]

mmm thanks for the help guys, but it seems my problem was just a complete lack of knowledge on the topic, and even your help is going over my head.
looks like i'll just flunk this question and hit the books so it doesn't happen again.

Hitting the books as a response to flunking a question shows wise judgement. Good luck. Tackling Laplace transforms w/o a knowledge of integration by parts is probably a poor idea. Just to tease, can you do the Laplace transform of 1? Differentiate it with respect to s.