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Find F(s) if: (a) f(t)=6etcosh2tf(t)=6 e^{-t} \cosh 2 t (b) f(t)=3te2tsinh4tf(t)=3 t e^{-2 t} \sinh 4 t (c) f(t)=8e3tcoshtu(t2)f(t)=8 e^{-3 t} \cosh t u(t-2)

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Answered 2 years ago
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(a)\textbf{(a)}

We can express f(t)f(t) as

f(t)=6et(e2t+e2t2)u(t)=3(et+e3t)u(t)\begin{align*} f(t)&=6e^{-t}\left(\frac{e^{2t}+e^{-2t}}{2}\right)u(t)\\ &=3\left(e^{t}+e^{-3t}\right)u(t) \end{align*}

By the linearity property,

F(s)=3L[etu(t)]+3L[e3tu(t)]=3(1s1+1s+3)=6(s+1)s2+2s3\begin{align*} F(s)&=3\mathcal{L}[e^{t}u(t)]+3\mathcal{L}[e^{-3t}u(t)]\\ &=3\left(\frac{1}{s-1}+\frac{1}{s+3}\right)\\ &=\boxed{\frac{6(s+1)}{s^2+2s-3}} \end{align*}

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