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Homework Help: A couple laplace transforms, need help

  1. Apr 14, 2007 #1
    1. The problem statement, all variables and given/known data
    just a small part of a large problem ><a im not even sure if this is in the right place. the first two questions i have to ask are really minor so i didnt want to make separate threads for each one. the third one is an actual problem itself tho.

    (1) i need the laplace inverse of :
    [s^2]/[s^2 - 3^2]
    -solved, there was an error at the start that threw the rest of my eqn off. there is no s^2

    (2) laplace transformation of:
    u(t-3)
    - solved

    (3) find laplace inverse of:
    F(s) = [3*e^(-4s)] / [s*(s^2 + s + 5/4)]


    2. Relevant equations
    for the second one, the u comes from working with time intervals using the second shift theorem
    which is: (L(g(t - k)u(t - k)) = e^(-k*s)*G(s)
    where G(s) is the laplace transform of of g(t)
    the third question also uses


    3. The attempt at a solution

    for the first one, i have no idea of how to even look at this because i've been using a laplace conversion sheet to solve them all up until now. the only laplace transformation i've seen with s^2 in the numerator looks just like the problem i have here but the intire denominator is squared awell.. other than that i cant find a laplace transform to matchit. -solved


    the second one seems simple enough but im stuck. it was originally part of another function i had to laplace, but it was too complicated to laplace as it was and i singled that much out. the original term was:

    (t - 2)u(t - 3) and so i broke this into smaller terms so thati could use the second shifting theorem on one of the termsand got:

    (t - 3)u(t - 3) + u(t - 3)
    [im leaving out the multiplication "*" signs so its easier to look at]

    then that laplaced to:

    e^(-3*s)/[s^2] + [the laplace of the second term]

    im not quite sure whether this just counts as one and laplaces to (1/s) or something.
    -solved

    for question three i took out the e and its power because the -4*s would be used for the second shifting theorem. leaving 3/[s*(s^2 + s + 5/4)]

    from here i tried to use partial fractions on the equation to make it easier, but i keep ending up getting 2 different values for my first variable.

    also with the (s^2 + s + 5/4) i perfected the square to:
    [(s + 1/2)^2 + 1]

    if i could work the partial fraction for this, i might beable to solve the laplace inverses for the single terms, using the laplace transformation chart. so the main problem is the partial fraction.
    i had to "un-perfect the square" to make this part easier

    what i have for that is:
    3/[s*(s^2 + s + 5/4)] = A/s + B*(s^2 + s + 5/4)
    3/[s*(s^2 + s + 5/4)] = [A*(s^2 + s + 5/4) + B*s] / [s*(s^2 + s + 5/4)]
    3 = A*(s^2 + s + 5/4) + B*s
    3 = A*s^2 + A*s + A*5/4 + B*s
    3 = A*s^2 + (A+B)*s + A*5/4

    this is what is giving me two separate values for A,
    where due to the first term, A must equal zero.
    but due to the second term A must equal to 12/5.

    thanks if you can help

    edit: i clicked off of the form without realising and accidentally submit. i think.
     
    Last edited: Apr 14, 2007
  2. jcsd
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