linear differential equations??

• Apr 19th 2008, 10:10 PM
iloveyou33
linear differential equations??
I have a few questions about finding the general solution of first-order linear differential equations..

http://i28.tinypic.com/2yv3343.gif

http://i31.tinypic.com/2818w02.gif

http://i28.tinypic.com/n81ee.gif
• Apr 19th 2008, 10:14 PM
Jhevon
Quote:

Originally Posted by iloveyou33
I have a few questions about finding the general solution of first-order linear differential equations..

http://i28.tinypic.com/2yv3343.gif

http://i31.tinypic.com/2818w02.gif

http://i28.tinypic.com/n81ee.gif

the integrating factor method takes care of all these. see post #21 here
• Apr 19th 2008, 10:20 PM
iloveyou33
it doesn't make sense though..
is there a simpler way of explaining it?
I don't understand what "the integrating factor" is in the first place..
• Apr 19th 2008, 10:33 PM
Jhevon
Quote:

Originally Posted by iloveyou33
it doesn't make sense though..
is there a simpler way of explaining it?
I don't understand what "the integrating factor" is in the first place..

it is just something you multiply through by to make the left hand side the derivative given by the product and the integrating factor. for the first one:

$\displaystyle y' + 2y = 3e^t$

(with practice, you will be able to recognize the integrating factor immediately, but let's go through the method to see it)

the integrating factor is $\displaystyle e^{\int 2~dt} = e^{2t}$

multiply through by the integrating factor, we get:

$\displaystyle e^{2t}y' + 2e^{2t}y = 3e^{3t}$

now the left hand side is the result of differentiating $\displaystyle e^{2t}y$ by the product rule, thus

$\displaystyle (e^{2t}y)' = 3e^{3t}$

integrate both sides, we get:

$\displaystyle e^{2t}y = e^{3t} + C$

$\displaystyle \Rightarrow y = e^t + Ce^{-2t}$

the others are done similarly
• Apr 20th 2008, 08:51 AM
Krizalid
Quote:

Originally Posted by Jhevon
the integrating factor method takes care of all these. see post #21 here

You always give that example, I think you may put a link in your signature with that. :D
• Apr 20th 2008, 09:44 AM
Jhevon
Quote:

Originally Posted by Krizalid
You always give that example, I think you may put a link in your signature with that. :D

Haha, yeah. It's kinda weird though. I do not want something in my signature as particular as a kind of differential equations problem. I want general things in my signature, like yours!