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MathGroup Archive 2001

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Re: desperate, please help.

  • To: mathgroup at smc.vnet.net
  • Subject: [mg27374] Re: desperate, please help.
  • From: "Allan Hayes" <hay at haystack.demon.co.uk>
  • Date: Thu, 22 Feb 2001 02:25:20 -0500 (EST)
  • References: <97003k$tq@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

There are several syntax errors, but even when they have been corrected the
equation is not solved.
However, algebraic equations are simply dealt with

rhs=-F/r[x]//.
    {
      T[x]->330+43.3*x,F->.9*163,C->9.3,
      k[x]->31.1Exp[7906*(T[x]-360)/(T[x]*360)],
      K[x]->3.03*Exp[-830.3*((T[x]-333)/(T[x]*333))],
      r[x]->-k[x]*C*(1-(1+1/K[x])*x)
      }

0.5072087957680738/
  (E^((3953*(-30 + 43.3*x))/(180*(330 + 43.3*x)))*
   (1 - (1 + 0.33003300330033003*
       E^((2.493393393393393*(-3 + 43.3*x))/
         (330 + 43.3*x)))*x))

With this the problem reduces to the following, which Mathematica quickly solves

NDSolve[{v'[x] == rhs, v[0] == 0}, {v}, {x, 0, .7}];

Look at the plot.

Plot[v[x] /. %, {x, 0, 0.7}]


--
Allan
---------------------
Allan Hayes
Mathematica Training and Consulting
Leicester UK
www.haystack.demon.co.uk
hay at haystack.demon.co.uk
Voice: +44 (0)116 271 4198
Fax: +44 (0)870 164 0565

"Thjqase" <thjqase at aol.com> wrote in message news:97003k$tq at smc.vnet.net...
> I need help numerically solving a differential equation coupled with
several
> algebraic equations. New to Mathematica. Here's code:
>
> NDSolve[{v'[x] == -F÷r, T[x] == 330 + 43.3*x, F = .9*163, C == 9.3,
>     k[x] == 31.1Exp(7906*(T - 360)/(T*360))
>     , K[x] == 3.03*Exp(-830.3*((T - 333)/(T*333))),
>     r[x] == -k*C*(1 - (1 + 1/K)*x),
>     v[0] == 0}, {v}
>   , {x, 0, .7}]
>
> Error code is:
>
> NDSolve::"deql": "The first argument must have both an equation and an \
> initial condition."
>
> Any suggestions? What am I doing wrong? My head is sore from banging it
against
> the wall.
>
> TIA.
>




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