- #1

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I figured out my Y(s)=(-3s+20)/(s^2-6s+12). All I need to do is take the inverse laplace of this but I can't figure it. I know I need to split it into two fractions, but after that I'm lost. I'd appreciate any help.

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- Thread starter andrewdavid
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- #1

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I figured out my Y(s)=(-3s+20)/(s^2-6s+12). All I need to do is take the inverse laplace of this but I can't figure it. I know I need to split it into two fractions, but after that I'm lost. I'd appreciate any help.

- #2

Pyrrhus

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Yes Mr Beagss, it's right.

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- #3

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Try doing complete the square on the bottom... see what happens

- #4

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- #5

Pyrrhus

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Ok now look at your Laplace Table of inverse and convert them.

For example

[tex] e^{at} \sin (bt) = \frac{b}{(s-a)^{2} + b^{2}} [/tex]

[tex]10 \frac{2}{(s-3)^{2} + (2)^{2}} = 10 e^{3t} \sin (2t) [/tex]

For example

[tex] e^{at} \sin (bt) = \frac{b}{(s-a)^{2} + b^{2}} [/tex]

[tex]10 \frac{2}{(s-3)^{2} + (2)^{2}} = 10 e^{3t} \sin (2t) [/tex]

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- #6

Pyrrhus

Homework Helper

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For the other Laplace inverse is:

[tex] e^{at} \cos (bt) = \frac{s-a}{(s-a)^{2} + b^{2}} [/tex]

[tex] \frac{-3s}{(s-3)^{2} + 4} = \frac{-3s + 9 - 9}{(s-3)^{2} + 4} [/tex]

thus

[tex] \frac{-3s + 9 - 9}{(s-3)^{2} + 4} = \frac{-3(s - 3) - 9}{(s-3)^{2} + 4} [/tex]

[tex] \frac{-3(s - 3) - 9}{(s-3)^{2} + 4} = \frac{-3(s - 3)}{(s-3)^{2} + 4} + \frac{-9}{(s-3)^{2} + 4} [/tex]

and finally

[tex] \frac{-3(s - 3)}{(s-3)^{2} + 4} = -3 e^{3t} \cos(2t) [/tex]

[tex] \frac{-9}{2} \frac{2}{(s-3)^{2} + (2)^{2}} = \frac{-9}{2} e^{3t} \sin (2t) [/tex]

so for the end Laplace inverse of

[tex] \frac{-3s}{(s-3)^{2} + 4} = -3 e^{3t} \cos(2t) + \frac{-9}{2} e^{3t} \sin (2t) [/tex]

[tex] e^{at} \cos (bt) = \frac{s-a}{(s-a)^{2} + b^{2}} [/tex]

[tex] \frac{-3s}{(s-3)^{2} + 4} = \frac{-3s + 9 - 9}{(s-3)^{2} + 4} [/tex]

thus

[tex] \frac{-3s + 9 - 9}{(s-3)^{2} + 4} = \frac{-3(s - 3) - 9}{(s-3)^{2} + 4} [/tex]

[tex] \frac{-3(s - 3) - 9}{(s-3)^{2} + 4} = \frac{-3(s - 3)}{(s-3)^{2} + 4} + \frac{-9}{(s-3)^{2} + 4} [/tex]

and finally

[tex] \frac{-3(s - 3)}{(s-3)^{2} + 4} = -3 e^{3t} \cos(2t) [/tex]

[tex] \frac{-9}{2} \frac{2}{(s-3)^{2} + (2)^{2}} = \frac{-9}{2} e^{3t} \sin (2t) [/tex]

so for the end Laplace inverse of

[tex] \frac{-3s}{(s-3)^{2} + 4} = -3 e^{3t} \cos(2t) + \frac{-9}{2} e^{3t} \sin (2t) [/tex]

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