Laplace Transforms (First Order DE with Initial Value)

  • Thread starter cas159
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  • #1
5
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Y' + 8y = e^-2t*sint, with initial condition y(0) = 0

Alright so I've been working on this one for about an hour, I really don't know why but I'm having major problems with these types of problems, whether i don't understand how to set it up or I don't understand partial fraction decomposition, I do not know, but this is what I have done.

(For reference, L{function} is the notation I will use).

L{y''} + 8L{y} = L{e^(-2t)*sin(t)}

sY(s) - y(0) + 8Y(s) = 1/((s+2)² + 1)

1/((s+2)² + 1) = 1/(s²+4s+5)

(s+8)Y(s) = 1/(s²+4s+5)

Y(s) = 1/((s²+4s+5)(s+8))

Now setting up my Partial Fraction Decomposition

As+B/(s²+4s+5) + C/(s+8) = 1/((s²+4s+5)(s+8))

As² + Bs + 8As + 8B + Cs² + 4Cs + 5c = 1

(A+C)s² + (8A + B + 4C)s + 8B + 5C = 1

Setting up my equations:

A + 0B + C = 0
8A + B + 4C = 0
0A + 8B + 5C = 1

I'm getting that A = -1/37, B = 4/37, and C = 1/37, the rest I can do, but the book says that B should be 6/37, can anyone figure out what I did improperly, I would really appreciate it.
 

Answers and Replies

  • #2
798
34
I agree with B=4/37
May be a typing mistake in the book. Did you compare the final results y(t) ?
 
  • #3
5
0
I did, and with a friend as well and he got 4/37 as well. Thanks a lot, books are so awesome sometimes.
 
  • #5
5
0
Okay sorry, I posted it in the one and read the rules and posted in the correct one (the homework one), am I allowed to delete a post? But yea, sorry.
 

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