Hello,
I need laplace transform of these function:
f(t)=(t^n)cos(a*t)
and
f(t)=(t^n)cosh(a*t)
For the first, note that
$\displaystyle \mathcal{L}\left[t^{n}f(t)\right]=(-1)^{n}\dfrac{d^{n}}{ds^{n}}\,F(s),$
and
$\displaystyle \mathcal{L}\left[\cos(at)\right]=\dfrac{s}{s^{2}+a^{2}}.$
For the second, note in addition that
$\displaystyle \mathcal{L}\left[\cosh(at)\right]=\dfrac{s}{s^{2}-a^{2}}.$
Where does that get you?