Originally Posted by

**mike10003** My problem is regarding this function:

$\displaystyle H(t-pi/2) * cos(3t)$

and Im suppose to write the Laplace transformation of this product using the theorem:

$\displaystyle

L[H(t-c) f(t-c)] = e^{-cs}*L(f)

$

I understand the principle behind this problem, and I know the answer is $\displaystyle e^{-pi*s/2}$ * L(cos(3t) in terms of t-pi/6). Its just I can't figure out how to rewrite $\displaystyle cos(3t)$ in terms of $\displaystyle t-pi/2$. I tried to do cos$\displaystyle (2t + t), $ use the addition property, and end up getting stuck with a $\displaystyle 1-4sin(t)^2$ that I dont know what to do with. Can someone help me out?