Answer
$- \dfrac{s \cdot e^{-\pi s}}{s^2+1}$
Work Step by Step
We are given that $f(t)=\cos (t) u_{\pi} (t)=-\cos (t-\pi) u_{\pi} (t)$
Now,$F(s)=L[-\cos (t-\pi) u_{\pi} (t)] \\= -e^{-\pi s} L [\cos t]=- \dfrac{s \cdot e^{-\pi s}}{s^2+1}$
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