Re: desperate, please help.

*To*: mathgroup at smc.vnet.net*Subject*: [mg27362] Re: desperate, please help.*From*: Jens-Peer Kuska <kuska at informatik.uni-leipzig.de>*Date*: Thu, 22 Feb 2001 02:25:07 -0500 (EST)*Organization*: Universitaet Leipzig*References*: <97003k$tq@smc.vnet.net>*Sender*: owner-wri-mathgroup at wolfram.com

Hi, Mathematica can't solve differential algebraic equations. More over that is a bit difficult because only for special input values a solution exist. Don't type stuff like ÷ into equations, don't make assignments like F = .9*163 in differential equations, don't mix the case in k[x] and K[x], use always an argument (not a single K or r) and try to elimimate T[x],F,C,K[x].. with Eliminate[] before you call NDSolve[]. Regards Jens Thjqase wrote: > > 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.