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I have the following expression to get its laplace transfer: $e^{2t}(3t-3t^2)$

Is it ok to just calculate the transfer of each term then multiply the result? I calculated the expression above like this but it is different than the answer in my book:

${\frac{1}{s-2}}*\frac{3}{s^2}-\frac{6}{s^3}$ $\frac{1}{s-2}*\frac{3s^3-6s^2}{s^5}$ $\frac{3s^3-6s^2}{(s-2)s^5}$

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    Aaaa I got the idea, thanks!2012-06-07

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We have

$e^{2t}(3t-3t^2)=3e^{2t}(t-t^2)$

$\mathcal{L}(t-t^2)=\mathcal{L}(t)-\mathcal{L}(t^2)=\frac{1}{s^2}-\frac{2}{s^3}$

You cannot simply multiply the transforms as you wanted (try it on $t^2=t *t$ as a counter example). However, using David Mitra's shift rule, we have

$3\mathcal{L}(e^{2t}(t-t^2))=3\left(\frac{1}{(s-2)^2}-\frac{2}{(s-2)^3}\right)=\frac{3(s-4)}{(s-2)^3}$