# Laplace transform

• April 19th 2010, 08:41 PM
snaes
Laplace transform
$cos(3t-1)$ I need to find the laplace transform of this.

As given, I cannot just use the table of given identities. I think i need to use some trig function to seperate the cosine, but I cannot seem to find anything that seems to help....

Is there anyway to seperate this into something a little easier?
Thanks!
• April 19th 2010, 09:08 PM
mr fantastic
Quote:

Originally Posted by snaes
$cos(3t-1)$ I need to find the laplace transform of this.

As given, I cannot just use the table of given identities. I think i need to use some trig function to seperate the cosine, but I cannot seem to find anything that seems to help....

Is there anyway to seperate this into something a little easier?
Thanks!

Use 2.16(b) from here: Table of Laplace Transforms

Note that $\cos (3t - 1) = \sin \left(\frac{\pi}{2} - [3t - 1]\right) = - \sin \left(3t - 1 - \frac{\pi}{2}\right)$.

(Or just do it from first principles by calculatiing the integral).
• April 23rd 2010, 02:06 AM
bandedkrait
$\cos(3t-1) = \cos(1)\cos(3t)+\sin(1)\sin(3t)$

Therefore $F(s) = \cos(1)L(\cos(3t))+\sin(1)L(\sin(3t))$
• April 23rd 2010, 06:48 AM
snaes
trig identities....they'll get you. Thanks