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# 2nd Order Differentials

Maths and statistics discussion, revision, exam and homework help.

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1. 2nd Order Differentials
Hey guys, bit stuck here. EDIT: And girls. Lets not be sexist here. Although we all know girls doing maths is like the cake.

d2y/dx2+4(dy/dx)+5y=65sin(2x)

y=(A+Bx)e^(-2x) + sin2x - 8cos2x

"Show that for large values of x this general solution may be approximated by a sine function and find this sine function"

Now as x -> infinity e^(-2x) -> 0, hence the first part drops out.

We now have y=sin2x - 8cos2x

How do we go from there to sqrt(65)sin(2x-c) where tan(c) = 8 ?? (This is teh answer given in the back of the book)

I haven't a clue what they've done there.

Any ideas?
Last edited by Xarren; 18-06-2012 at 23:35.
2. Re: 2nd Order Differentials
What cake?
3. Re: 2nd Order Differentials
(Original post by TenOfThem)
What cake?
Portal style cake.

The type of cake that's always a lie.
4. Re: 2nd Order Differentials
Since you're dealing with 2x, see if you can match R*sin(2x+a) to sin2x - 8cos2x. Sometimes you might be asked for something in the form of R*cos(bx+a) but that's similar, just expand out and match.
5. Re: 2nd Order Differentials
You use Rsin(2x+a)
6. Re: 2nd Order Differentials
(Original post by Xarren)
Portal style cake.

The type of cake that's always a lie.
I will try to avoid answering any more questions from you then
7. Re: 2nd Order Differentials
(Original post by TenOfThem)
I will try to avoid answering any more questions from you then
I'm only kidding, don't get your nuts in a twist. Oh, wait.. :P

And yeah - I completely forgot about that. C3 feels like another life ago.

I honestly don't know a single girl that does FM. Like in real life that is
8. Re: 2nd Order Differentials
(Original post by Xarren)
I'm only kidding, don't get your nuts in a twist. Oh, wait.. :P

And yeah - I completely forgot about that. C3 feels like another life ago.

I honestly don't know a single girl that does FM. Like in real life that is
I teach it

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Last updated: June 18, 2012
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