Laplace transformation

1. May 15, 2014

skrat

1. The problem statement, all variables and given/known data
Find Laplace transformation for functions $f(t)$:
a) $5cos(7t+\pi /4)$
b) $e^{3t}sintcost$

2. Relevant equations

3. The attempt at a solution

a) I know that for $cos(\omega t)$ the laplace is $\frac{s}{s^2+\omega ^2}$ but what can I do with that $\pi /4$ ?

I believe I would have to use this identity $f(t)^{'}=sF(s)+f(0)$ but I don't know how o use it...

Could somebody please show me that?

2. May 15, 2014

LCKurtz

Use the addition formula for $\cos(a+b)$ before taking the transform.

3. May 15, 2014

skrat

OMG, How can I be so stupid! -.-

Thanks LCKurtz!