Laplace Transform for Functions: 5cos(7t+π/4) and e^(3t)sintcost

[skrat]

Homework Statement

Find Laplace transformation for functions $f(t)$:
a) $5cos(7t+\pi /4)$
b) $e^{3t}sintcost$

Homework Equations

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?

 

[LCKurtz]

Homework Statement

Find Laplace transformation for functions $f(t)$:
a) $5cos(7t+\pi /4)$
b) $e^{3t}sintcost$

Homework Equations

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?

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

 

[skrat]

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

Thanks LCKurtz!