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Differential equations

  • Thread starter glid02
  • Start date
54
0
Here's the problem:
Solve y'-4y=9e^(7t) with y(0)=5

p(t)=-4
mu(t)=e^(int[-4dt])=e^(-4t)

multiplying both sides by mu(t)
e^(-4t)(y'-4y)=9e^(3t)
Dt(e^(-4t)y)=9e^(3t)
e^(-4t)y=3e^(3t)+c
y=(3e^(3t))/(e^(-4t))+c/e^(-4t)

now when y(0)=5
5=3+c
c=2

so y=(3e^(3t))/(e^(-4t))+2

That's not right, I also tried y=(3e^(3t))/(e^(-4t))(5/3)
and that's not right either. Any idea what I'm doing wrong?

Thanks.

Never mind, figured it out, I didn't divide c by the left side of the equation.
 
Last edited:

Answers and Replies

13
0
your on the right track. You have the correct integration factor.
[tex] \mu(t)= e^{-4t} [tex]
 
13
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hmmm, still havent gotten the hang of how to type formulas here. Ok lets if I get it right this time....
After you set up the problem you actually need to do a full integration on the RHS, and you need to evaluate your LFS at the endpoints of integration...
[tex] \frac{d}{dt} y(t)e^{-4t}= 9e^{3t} \rightarrow y(t)e^(-4t') \Big^t_0= \int^t_0 9e^{3t'}dt' [\tex]
 
13
0
ok sorry , see if i can get it now....
[tex] \frac{d}{dt} y(t)e^{-4t}= 9e^{3t} \rightarrow y(t)e^(-4t') \Big^t_0= \int^t_0 9e^{3t'}dt' [/tex]
 
13
0
last try....
[tex]\frac{d}{dt} \left[y(t)e^{-4t} \right]= 9e^{3t} \rightarrow y(t)e^{-4t'} \Big| _0^t= \int^t_0 9e^{3t'}dt'[/tex]
 
Last edited:
HallsofIvy
Science Advisor
Homework Helper
41,732
893
Why do you have primes on the ts? On the right hand side, yes, the t inside the integral is a "dummy" variable and so should be distinguished from the t outside. I would be inclined to use a completely different letter, say s. There certainly shouldn't be a prime on the t in the exponential on the left.
To finish, what is
[tex]\int_0^t 9e^{3s}ds[/tex]
 

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