Differential equation help

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Differential equation help

Postby seladore » Thu Apr 16, 2009 9:47 am UTC

Hi all,

I've got a second order D.E., and the way that I would normally solve it gives an answer different to what I think I should get.
[math]\ddot{\phi} = c\phi[/math]
Where c is some combined constant. So, you just solve the auxiliary equation,
[math]k^2 - c = 0[/math]
[math]k_1 = \sqrt{c},\;\; k_2 = - \sqrt{c}[/math]
[math]\phi = Ae^{\sqrt{c}x} + Be^{-\sqrt{c}x}[/math]

Is the answer I'd get. But the answer I'm supposed to be getting only has the [imath]Be^{-\sqrt{c}x}[/imath] term, and I can't see how you would lose the other bit. Am I doing it totally wrong?

Any help would be appreciated.

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Re: Differential equation help

Postby madprocess » Thu Apr 16, 2009 10:42 am UTC

Do you have any initial conditions?

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