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Re: Re: Wrong ODE solution in Mathematica 7?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg106243] Re: [mg106194] Re: [mg106177] Wrong ODE solution in Mathematica 7?
  • From: DrMajorBob <btreat1 at austin.rr.com>
  • Date: Wed, 6 Jan 2010 05:58:26 -0500 (EST)
  • References: <201001041058.FAA21179@smc.vnet.net>
  • Reply-to: drmajorbob at yahoo.com

Actually, the difference is a constant plus another constant times x.

Bobby

On Tue, 05 Jan 2010 00:42:00 -0600, Tony Harker <a.harker at ucl.ac.uk> wrote:

> Let Mathematica check it:
>
> eq = D[y[x], x, x] == -Cos[x]/(1 + Sin[x])^2
> sol = DSolve[eq, y, x]
> eq /. sol[[1]] // Simplify
>
>  returns True.
>
>  So the solution is correct, and if you think about it the difference
> between Mathematica's solution and the other one is only a matter of a
> difference in the additive constant.
>
>   Tony
>
> ]-> -----Original Message-----
> ]-> From: Zsolt [mailto:phyhari at gmail.com]
> ]-> Sent: 04 January 2010 10:59
> ]-> To: mathgroup at smc.vnet.net
> ]-> Subject: [mg106177] Wrong ODE solution in Mathematica 7?
> ]->
> ]-> Hi!
> ]-> I tried solve the ODE:
> ]-> DSolve[D[y[x], x, x] == -Cos[x]/(1 + Sin[x])^2, y[x], x]
> ]->
> ]-> The solution what M7 (and Wolfram Alpha) gives is:
> ]-> y[x] -> C[1] + x C[2] + (2 Sin[x/2])/(Cos[x/2] + Sin[x/2])
> ]->
> ]-> I think, it's wrong! (Does anybody know how to check?)
> ]-> Another system gives for the same diff.eq:
> ]-> y(x) = -2/(tan((1/2)*x)+1)+_C1*x+_C2
> ]-> (similar, but not the same->ctan vs tan...) I found the
> ]-> problem in one of my math books, and the solution there
> ]-> concours with the other system.
> ]-> How can I trust Mathematica, if it makes mistakes in such
> ]-> simple things?? :( Thank you for your answer! :)
> ]->
> ]->
>
>


-- 
DrMajorBob at yahoo.com


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