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Re: Help with differential equation

  • To: mathgroup at smc.vnet.net
  • Subject: [mg112267] Re: Help with differential equation
  • From: Valeri Astanoff <astanoff at gmail.com>
  • Date: Tue, 7 Sep 2010 02:02:33 -0400 (EDT)
  • References: <i627tc$b9$1@smc.vnet.net>

On 6 sep, 10:14, mathpavi <elvisgracel... at gmail.com> wrote:
> Dear experts,
> I tried solving  the following differential equation using DSolve:
>
> y' ' [x]+a( y' [x] ) = b( cos[y] )
>
> But I got error message which asked me to provide proper arguments for
> cos..
> So I changed cos[y] to cos[kx] and obtained solution...
> However, I have to solve the equation by keeping cos[y] in order to
> obtain a solution of relevance to my work.
> Please help.
> praveen

You cant DSolve it
You better use NDSolve with initial conditions
For example :

f[x_] = y[x] /.
  First@NDSolve[
    y[0] == 0 && y'[0] == 0 && y''[x] + 2 (y'[x]) == 3 (Cos[y[x=
]]),
    y[x], {x, 0, 4 Pi}]


 Plot[f[x], {x, 0, 4 Pi}]



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