# Inverse Laplace transform

1. May 17, 2009

### tony873004

L-1(1/(s+2)3)
I don't see this one in the table. How do I solve the inverse Laplace transform?

I know from class notes that the answer is (1/2) t2e-2t
But I don't know to get it.
Thanks!
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. May 17, 2009

### gabbagabbahey

I assume that $$\mathcal{L}^{-1}\left[ \frac{1}{s^3} \right]$$ is in your table?

If so, just begin by applying the frequency shift rule:

$$\mathcal{L}^{-1}\left[ f(s+a) \right]=e^{-at}\mathcal{L}^{-1}\left[ f(s) \right]$$

3. May 17, 2009

### tony873004

ok, thanks. That gives me
$$\frac{1}{(e^{-2t}L^{-1}(s))^3}$$

And the table gives for $$L^{-1}\frac{1}{s^3}$$ as $$\frac{t^2}{2}$$

What do I do from here?

4. May 17, 2009

### gabbagabbahey

How do you get that ugly expression?

You should get

$$\mathcal{L}^{-1}\left[ \frac{1}{(s+2)^3} \right]=e^{-2t}\mathcal{L}^{-1}\left[ \frac{1}{s^3} \right]$$