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Homework Help: Inverse Laplace Transformations

  1. Oct 24, 2007 #1
    1. The problem statement, all variables and given/known data
    Find the inverse Laplace transform of the given functions:
    3. [tex]\frac{2}{s^2+3s-4}[/tex]

    7. [tex]\frac{2s+1}{s^2-2s+2}[/tex]

    2. Relevant equations
    Inverse Laplace Transform Table

    3. The attempt at a solution
    on 3. i made the denominator look like [tex](s+4)(s-1)[/tex] but i got lost from there. i couldn't find anything on the table resembling the equation.

    on 7. i completed the square and got [tex]\frac{2s+1}{(s-1)^2+1}[/tex], which resembles [tex]\frac{s-a}{(s-a)^2+b^2}[/tex] on the table, yet i can't seem to make the numerator look like [tex](s-a)[/tex].

    if it helps anyone, the answers are 3. [tex]\frac{2}{5}e^t-\frac{2}{5}e^{-4}[/tex] and 7. [tex]2e^{t}cos(t)+3e^{t}sin(t)[/tex].

    thanks in advance.

    edit: p.s. if anyone has a better table of inverse laplace transformations, the one i posted is really hard to read. i'm using the one in my book but it might be useful, if you need to refer to one in a table when trying to help, to refer to a table you're more comfortable reading.
  2. jcsd
  3. Oct 24, 2007 #2


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    Science Advisor
    Homework Helper

    For the first one, use partial fractions to write the problem as A/(s-1)+B/(s+4) (find the constants A and B). Break up the second fraction as well. Write it as 2(s-1)/((s-1)^2+1) +C/((s-1)^2+1) (find the constant C). Use the linearity of the laplace transform.
    Last edited: Oct 24, 2007
  4. Oct 24, 2007 #3
    thank you very much. i'm really bad with partial fractions, so i guess my brain sort of tried to not think of that method. i guess i'll touch up on them because this chapter is filled with inverse laplace transformations.
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