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Laplace transformation to solve intial value problem

  • Thread starter Karmel
  • Start date
  • #1
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Homework Statement


laplace transformation to solve intial value problem
y"-2y+y=-4e^1-t + 2t
y(1)=0, y'(1)=-3


Homework Equations





The Attempt at a Solution


K. I have one more on the review that I am unaware of where I need to start. So if anyone can point me in the right direction please feel free. My graduation depends on the final and I am so lost on how to start amny of these problems.... Thanks in advance
 

Answers and Replies

  • #2
rock.freak667
Homework Helper
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Take the Laplace transformation on both sides noting that L{y(t)}=Y(s), then just make Y(s) the subject, then find the inverse laplace transform of Y(s) and you will the solution.
 
  • #3
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okay so I found the laplace of both sides and get

2/s^3 + 2/s^2 + 1/s = (-4e(-t+1)/s) + 2t/s

I'm not really sure where to go form here. I think that I want to set
Y=L{y(t)}(s)
L{y'}=sY
L{y"}=s^2Y

but I am not so sure that is the direction I need to head and I really am not sure were all this fits into the problem. Where do I head from here????
 
  • #4
rock.freak667
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L{y(t)}=sY(s)-Y'(0)

L{y''(t)}=s2Y(s)-sY(0)-Y'(0)

L{ekt}= 1/(s-k)

L{tn}=n!/sn+1
 
  • #5
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ok I get the first two but why the second two in the above reply. I guess I don't know what I need to do with them...
 
  • #6
rock.freak667
Homework Helper
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31
On looking back to the question, you need to change the variable such that the conditions occur when t=0. To do this we let t=w+1

so now we have

y''(w-1)-2y'(w-1)+y(w+1)=4e-1-w-1+2(w+1)

Now define u(w)=y(w+1)
so now u'(w)=y'(w+1) and u''(w)=y''(w+1)

Now when w=0 => u(0)=y(0+1)=y(1)=0 AND u'(w)=y'(1)=-3

so now you need so solve the new equation

u''-2u'+u=4e-w+2w+2


Now take the Laplace transform on both sides of the equation.

to elaborate on the second two...


if you need to get L{e5t} it is simplt 1/(s-5)

and L{t2}=2!/s3
 
  • #7
12
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Okay so now I get
L{u"}(s) = s2U(s) + 3
L{u'}(s) = sU(s)


u''-2u'+u=4e-w+2w+2

taking the LT of both sides I get
s2U(s)+3-2sU(s)+U(s)=(4/s+1)+(2/s2)+(2/s)
(s2-2s+1)U(s)=(4/s+1)+(2/s2)+(2/s)-3

I feel like I have messed up somewhere cause if I solve for U(s) then it gets really ugly....
 

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