So I'm to find $\displaystyle \mathcal{L}^{-1}\{e^{-\pi}}/(s^{2}+1)\}$ and I figure it should just be: The exponential tells me its unit step-functioned over by $\displaystyle \pi$ and then just Laplacian inverse the leftover so the result is $\displaystyle \sin(t-\pi) \mathcal{U}(t-\pi)$. My book says it's $\displaystyle -\sin t \mathcal{U}(t-\pi)$ and I don't get it.